common/rc: Add a new _require_scratch_extsize helper function

common/rc: Add a new _require_scratch_extsize helper function

_require_scratch_extsize helper function will be used in the
the next patch to make the test run only on filesystems with
extsize support.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Signed-off-by: Nirjhar Roy <nirjhar@linux.ibm.com>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>