xfs: move xfs_bmap_rtalloc to xfs_rtalloc.c

xfs: move xfs_bmap_rtalloc to xfs_rtalloc.c

xfs_bmap_rtalloc is currently in xfs_bmap_util.c, which is a somewhat
odd spot for it, given that is only called from xfs_bmap.c and calls
into xfs_rtalloc.c to do the actual work.  Move xfs_bmap_rtalloc to
xfs_rtalloc.c and mark xfs_rtpick_extent xfs_rtallocate_extent and
xfs_rtallocate_extent static now that they aren't called from outside
of xfs_rtalloc.c.

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