common: make rt_ops local in _try_scratch_mkfs_sized

common: make rt_ops local in _try_scratch_mkfs_sized

If we call _try_scratch_mkfs_size with $SCRATCH_RTDEV set followed by
a call with $SCRATCH_RTDEV cleared, rt_ops will have stale size
parameters that will cause mkfs.xfs to fail with:
"size specified for non-existent rt subvolume"

Make rt_ops local to fix this.

Signed-off-by: Hans Holmberg <hans.holmberg@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>