lockd: fix vfs_test_lock() calls

lockd: fix vfs_test_lock() calls

Usage of vfs_test_lock() is somewhat confused.  Documentation suggests
it is given a "lock" but this is not the case.  It is given a struct
file_lock which contains some details of the sort of lock it should be
looking for.

In particular passing a "file_lock" containing fl_lmops or fl_ops is
meaningless and possibly confusing.

This is particularly problematic in lockd.  nlmsvc_testlock() receives
an initialised "file_lock" from xdr-decode, including manager ops and an
owner.  It then mistakenly passes this to vfs_test_lock() which might
replace the owner and the ops.  This can lead to confusion when freeing
the lock.

The primary role of the 'struct file_lock' passed to vfs_test_lock() is
to report a conflicting lock that was found, so it makes more sense for
nlmsvc_testlock() to pass "conflock", which it uses for returning the
conflicting lock.

With this change, freeing of the lock is not confused and code in
__nlm4svc_proc_test() and __nlmsvc_proc_test() can be simplified.

Documentation for vfs_test_lock() is improved to reflect its real
purpose, and a WARN_ON_ONCE() is added to avoid a similar problem in the
future.

Reported-by: Olga Kornievskaia <okorniev@redhat.com>
Closes: https://lore.kernel.org/all/20251021130506.45065-1-okorniev@redhat.com
Signed-off-by: NeilBrown <neil@brown.name>
Fixes: 20fa19027286 ("nfs: add export operations")
Cc: stable@vger.kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>