xfstests: Obey mkfs options for sized filesystems on XFS

xfstests: Obey mkfs options for sized filesystems on XFS

The XFS implementation of _scratch_mkfs_sized ignores MKFS_OPTIONS
when a custom block size is set and so isn't testing things like
CRCs on such sized filesytsems. Fix this by ensuring we don't try to
override the block size is it is set in MKFS_OPTIONS. xfs/204 shows
this problem.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>