NFSD: Release the export reference when reaping open stateids

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

NFSD: Release the export reference when reaping open stateids

nfs4_put_stid() releases the svc_export tracked in
nfs4_stid.sc_export, but free_ol_stateid_reaplist() frees open and
lock stateids by calling ->sc_free() directly, bypassing that path.
An open stateid takes an sc_export reference in nfs4_open() and a
lock stateid takes its own in init_lock_stateid(); both reach
free_ol_stateid_reaplist() through their normal teardown, the open
stateid via release_open_stateid() and the lock stateid via
nfsd4_release_lockowner(), each through put_ol_stateid_locked().
The reference is therefore never dropped, pinning the export and
blocking unmount for the lifetime of the stateid.

Release sc_export in free_ol_stateid_reaplist() the way
nfs4_put_stid() does. ->sc_free() runs once per stateid, and a
stateid reaches free_ol_stateid_reaplist() or nfs4_put_stid() but
never both, so the reference is dropped exactly once. Revoked
stateids reach this path with sc_export already cleared by
drop_stid_export(), so they are skipped rather than double-freed.

nfs4_put_stid() itself read sc_export before acquiring cl_lock.
drop_stid_export() clears that field and releases the reference
under cl_lock, so a concurrent revocation could drop the export in
the window between the read and the final put, releasing the same
reference twice. Read sc_export while cl_lock is held so the two
paths serialize and the reference is released exactly once.