nfsd: rework refcounting in filecache

Bugzilla: https://bugzilla.redhat.com/2138605

commit ac3a258
Author: Jeff Layton <jlayton@kernel.org>
Date:   Sun Dec 11 06:19:33 2022 -0500

    nfsd: rework refcounting in filecache

    The filecache refcounting is a bit non-standard for something searchable
    by RCU, in that we maintain a sentinel reference while it's hashed. This
    in turn requires that we have to do things differently in the "put"
    depending on whether its hashed, which we believe to have led to races.

    There are other problems in here too. nfsd_file_close_inode_sync can end
    up freeing an nfsd_file while there are still outstanding references to
    it, and there are a number of subtle ToC/ToU races.

    Rework the code so that the refcount is what drives the lifecycle. When
    the refcount goes to zero, then unhash and rcu free the object. A task
    searching for a nfsd_file is allowed to bump its refcount, but only if
    it's not already 0. Ensure that we don't make any other changes to it
    until a reference is held.

    With this change, the LRU carries a reference. Take special care to deal
    with it when removing an entry from the list, and ensure that we only
    repurpose the nf_lru list_head when the refcount is 0 to ensure
    exclusive access to it.

    Signed-off-by: Jeff Layton <jlayton@kernel.org>
    Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

Signed-off-by: Jeffrey Layton <jlayton@redhat.com>

Author: jtlayton