container freezer: rename check_if_frozen()

check_if_frozen() sounds like it should return something when in fact it's
just updating the freezer state.

Signed-off-by: Matt Helsley <matthltc@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>