mkfs: fix reflink/rmap logic w.r.t. realtime devices and crc=0 support

mkfs: fix reflink/rmap logic w.r.t. realtime devices and crc=0 support

mkfs has some logic to deal with situations where reflink or rmapbt are
turned on and the administrator has configured a realtime device or a V4
filesystem; such configurations are not allowed.

The logic ought to disable reflink and/or rmapbt if they're enabled due
to being the defaults, and it ought to complain and abort if they're
enabled because the admin explicitly turned them on.

Unfortunately, the logic here doesn't do that and makes no sense at all
since usage() exits the program.  Fix it to follow what everything else
does.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>