libfs: Re-arrange locking in offset_iterate_dir()

libfs: Re-arrange locking in offset_iterate_dir()

Liam and Matthew say that once the RCU read lock is released,
xa_state is not safe to re-use for the next xas_find() call. But the
RCU read lock must be released on each loop iteration so that
dput(), which might_sleep(), can be called safely.

Thus we are forced to walk the offset tree with fresh state for each
directory entry. xa_find() can do this for us, though it might be a
little less efficient than maintaining xa_state locally.

We believe that in the current code base, inode->i_rwsem provides
protection for the xa_state maintained in
offset_iterate_dir(). However, there is no guarantee that will
continue to be the case in the future.

Since offset_iterate_dir() doesn't build xa_state locally any more,
there's no longer a strong need for offset_find_next(). Clean up by
rolling these two helpers together.

Suggested-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Message-ID: <170785993027.11135.8830043889278631735.stgit@91.116.238.104.host.secureserver.net>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Link: https://lore.kernel.org/r/170820142021.6328.15047865406275957018.stgit@91.116.238.104.host.secureserver.net
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>