Fix bug in sync control in new "dev" mode of ZVOL (r265678).

Don't check ZVOL_WCE flag, used in Solaris to control device "write cache".
It is not applicable on FreeBSD and by default set to "disable".

Ticket:	#5425

(cherry picked from commit 4618d6ea2b194495e611c1e2ccebc58e7bcd616d)