mkfs: reject cowextsize after making final decision about reflink support

mkfs: reject cowextsize after making final decision about reflink support

There's a snippet of code that rejects cowextsize option if reflink is
disabled.  This really ought to be /after/ the last place where we can
turn off reflink.  Fix it so that people don't see stuff like this:

$ mkfs.xfs -r rtdev=b.img a.img -f -d cowextsize=16
illegal CoW extent size hint 16, must be less than 9600.

(reflink isn't supported when realtime is enabled)

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>