xfs: move remote symlink target read function to libxfs

Move xfs_readlink_bmap_ilocked to xfs_symlink_remote.c so that the
swapext code can use it to convert a remote format symlink back to
shortform format after a metadata repair.  While we're at it, fix a
broken printf prefix.

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