libxfs: pass a transaction context through listxattr

libxfs: pass a transaction context through listxattr

Pass a transaction context so that a new caller can walk the attr names
and query the values all in one go without deadlocking on nested buffer
access.

While we're at it, make the existing xfs_repair callers try to use
empty transactions so that we don't deadlock on cycles in the xattr
structure.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Dave Chinner <dchinner@redhat.com>