kfifo: kfifo_is_{full,empty} should return bools, not ints

For consistency with other kfifo routines, return bool, not int.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Cc: Stefani Seibold <stefani@seibold.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>