xfs: introduce xfs_alloc_vextent_exact_bno()

xfs: introduce xfs_alloc_vextent_exact_bno()

Source kernel commit: 5f36b2ce79f254dd00cdc88374271df7ce843d56

Two of the callers to xfs_alloc_vextent_this_ag() actually want
exact block number allocation, not anywhere-in-ag allocation. Split
this out from _this_ag() as a first class citizen so no external
extent allocation code needs to care about args->type anymore.

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