xfs: use xfs_alloc_vextent_start_bno() where appropriate

Change obvious callers of single AG allocation to use
xfs_alloc_vextent_start_bno(). Callers no long need to specify
XFS_ALLOCTYPE_START_BNO, and so the type can be driven inward and
removed.

While doing this, also pass the allocation target fsb as a parameter
rather than encoding it in args->fsbno.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>