block: relax atomic write boundary vs chunk size check

block: relax atomic write boundary vs chunk size check

blk_validate_atomic_write_limits() ensures that any boundary fits into
and is aligned to any chunk size.

However, it should also be possible to fit the chunk size into any
boundary. That check is already made in
blk_stack_atomic_writes_boundary_head().

Relax the check in blk_validate_atomic_write_limits() by reusing (and
renaming) blk_stack_atomic_writes_boundary_head().

Signed-off-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>