kfifo: fix kfifo_alloc() to return a signed int value

Add a new __kfifo_int_must_check_helper() helper function, which is needed
for kfifo_alloc() to return the right signed integer value.

The origin __kfifo_must_check_helper() helper was renamed into
__kfifo_uint_must_check_helper() to show the sign which is expected and
returned.

(And revert the temporary disabling of __kfifo_must_check_helper())

Signed-off-by: Stefani Seibold <stefani@seibold.net>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>