xfs: push the calls to xfs_rtallocate_range out to xfs_bmap_rtalloc

xfs: push the calls to xfs_rtallocate_range out to xfs_bmap_rtalloc

Currently the various low-level RT allocator functions call into
xfs_rtallocate_range directly, which ties them into the locking protocol
for the RT bitmap.  As these helpers already return the allocated range,
lift the call to xfs_rtallocate_range into xfs_bmap_rtalloc so that it
happens as high as possible in the stack, which will simplify future
changes to the locking protocol.

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