btrfs: fix nossd and ssd_spread mount option regression

The commit

07802534 btrfs: Cleanup the btrfs_parse_options for remount.

broke ssd options quite badly; it stopped making ssd_spread
imply ssd, and it made "nossd" unsettable.

Put things back at least as well as they were before
(though ssd mount option handling is still pretty odd:
# mount -o "nossd,ssd_spread" works?)

Reported-by: Roman Mamedov <rm@romanrm.net>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Chris Mason <clm@fb.com>