netfs: Fix missing xas_retry() calls in xarray iteration

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

netfs: Fix missing xas_retry() calls in xarray iteration

netfslib has a number of places in which it performs iteration of an xarray
whilst being under the RCU read lock. It *should* call xas_retry() as the
first thing inside of the loop and do "continue" if it returns true in case
the xarray walker passed out a special value indicating that the walk needs
to be redone from the root[*].

Fix this by adding the missing retry checks.

[*] I wonder if this should be done inside xas_find(), xas_next_node() and
suchlike, but I'm told that's not an simple change to effect.

This can cause an oops like that below. Note the faulting address - this
is an internal value (|0x2) returned from xarray.

BUG: kernel NULL pointer dereference, address: 0000000000000402
...
RIP: 0010:netfs_rreq_unlock+0xef/0x380 [netfs]
...
Call Trace:
netfs_rreq_assess+0xa6/0x240 [netfs]
netfs_readpage+0x173/0x3b0 [netfs]
? init_wait_var_entry+0x50/0x50
filemap_read_page+0x33/0xf0
filemap_get_pages+0x2f2/0x3f0
filemap_read+0xaa/0x320
? do_filp_open+0xb2/0x150
? rmqueue+0x3be/0xe10
ceph_read_iter+0x1fe/0x680 [ceph]
? new_sync_read+0x115/0x1a0
new_sync_read+0x115/0x1a0
vfs_read+0xf3/0x180
ksys_read+0x5f/0xe0
do_syscall_64+0x38/0x90
entry_SYSCALL_64_after_hwframe+0x44/0xae

Changes:
========
ver #2)
- Changed an unsigned int to a size_t to reduce the likelihood of an
overflow as per Willy's suggestion.
- Added an additional patch to fix the maths.