s390/fpu: change type of fpu mask from u32 to int

Change type of fpu mask consistently from u32 to int. This is a
prerequisite to make the kernel fpu usage preemptible. Upcoming code
uses __atomic* ops which work with int pointers.

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>