xfs: move xfs_ioc_getfsmap out of xfs_ioctl.c

Move this function out of xfs_ioctl.c to reduce the clutter in there,
and make the entire getfsmap implementation self-contained in a single
file.

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