unionfs: fix suspendability check bugs

• MNTK_SUSPENDABLE is set in mnt_kern_flag, not mnt_flag.
• The lower layer of a unionfs mount is read-only, so the mount should be suspendable iff the upper layer is suspendable.
• Remove a couple of superfluous comments.

Differential Revision: https://reviews.freebsd.org/D2714
Reviewed by: kib, mjg