libfrog: add a function to grab the path from an open fd and a file handle

libfrog: add a function to grab the path from an open fd and a file handle

handle_walk_paths operates on a file handle, but requires that the fs
has been registered with libhandle via path_to_fshandle.  For a normal
libhandle client this is the desirable behavior because the application
*should* maintain an open fd to the filesystem mount.

However for xfs_healer this isn't going to work well because the healer
mustn't pin the mount while it's running.  It's smart enough to know how
to find and reconnect to the mountpoint, but libhandle doesn't have any
such concept.

Therefore, alter the libfrog getparents code so that xfs_healer can pass
in the mountpoint and reconnected fd without needing libhandle.  All
we're really doing here is trying to obtain a user-visible path for a
file that encountered problems for logging purposes; if it fails, we'll
fall back to logging the inode number.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>