param: fix module param locks when !CONFIG_SYSFS.

As Dan Streetman points out, the entire point of locking for is to
stop sysfs accesses, so they're elided entirely in the !SYSFS case.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>