ceph: fix hanging __ceph_get_caps() with stale mds_wanted

In the Linux kernel, the following vulnerability has been resolved:

ceph: fix hanging __ceph_get_caps() with stale mds_wanted

A reader can hang forever in __ceph_get_caps() when the client no
longer holds `FILE_RD`, but local cap state still says that the
capability is already wanted (via `mds_wanted`).

One way to trigger this is through MDS cap revocation. If another
client performs a conflicting operation, the MDS can revoke `FILE_RD`
from the reader; the next read then has to reacquire `FILE_RD`. If
the cap update that should request `FILE_RD` never reaches the MDS
after `cap->mds_wanted` was raised, the reader is left holding only
non-file caps while local `mds_wanted` still includes the file read
caps.

In that state, try_get_cap_refs() sees `need <= mds_wanted` and
returns 0, so __ceph_get_caps() just waits on `i_cap_wq`. If the cap
update that was supposed to request `FILE_RD never reaches the MDS
after `cap->mds_wanted was` raised, no further request is sent and the
waiter can sleep indefinitely until unrelated cap traffic happens to
wake it up.

The ordering issue is that `cap->mds_wanted` is updated in
__prep_cap() before the `CEPH_MSG_CLIENT_CAPS message` is actually
queued for send. That makes one field serve two different meanings at
once: what this client wants, and what the client believes the MDS
already knows it wants.

A proper fix would be to split those states and track whether a cap
update is actually in flight or has been observed by the MDS.
However, simply moving the `cap->mds_wanted assignment` later would
not be sufficient: queueing the message in the messenger does not
guarantee that the MDS processed that specific wanted set, and
reconnect or message loss can still invalidate that assumption.
Fixing that properly would require a larger rework of the cap state
machine.

To allow simpler backports to stable kernels, this patch implements a
simpler workaround:

- stop waiting forever in __ceph_get_caps(); after a bounded wait,
fall back to the renew path

- make ceph_renew_caps() issue a synchronous `OPEN` request whenever
the inode still does not actually hold the wanted caps, instead of
only calling ceph_check_caps()

The extra issued-vs-wanted check in ceph_renew_caps() is necessary
because the previous test only checked whether the inode still had any
real caps at all. That is not enough after revocation: the client can
still hold something like `pLs` and yet be missing `FILE_RD`
completely. In that case, falling back to ceph_check_caps() is not
sufficient, because it still trusts `cap->mds_wanted` and may resend
nothing. By requiring `(issued & wanted) == wanted` before taking the
asynchronous path, the code only uses ceph_check_caps() when the
`wanted caps` are already actually issued. Otherwise, it sends the
synchronous `OPEN` renew.

This preserves the existing asynchronous fast path when the wanted
caps are already issued, avoids changing cap-state semantics, and
fixes the hang by guaranteeing that a stalled waiter eventually
retries through a path that does not rely on the stale `mds_wanted`
state.

[ idryomov: move CEPH_GET_CAPS_WAIT_TIMEOUT from libceph.h to
mds_client.h, formatting ]