[S390] fix hypfs build failure

Fix build breakage below which probably was introduced with
("rcu: don't include unnecessary headers, allow kmemtrace w/ tracepoints").

  CC      arch/s390/hypfs/hypfs_diag.o
arch/s390/hypfs/hypfs_diag.c: In function 'diag204_free_buffer':
arch/s390/hypfs/hypfs_diag.c:364: error: implicit declaration of function 'free_pages'
arch/s390/hypfs/hypfs_diag.c: In function 'diag204_alloc_rbuf':
arch/s390/hypfs/hypfs_diag.c:384: error: implicit declaration of function '__get_free_pages'
arch/s390/hypfs/hypfs_diag.c:384: error: 'GFP_KERNEL' undeclared (first use in this function)
arch/s390/hypfs/hypfs_diag.c:384: error: (Each undeclared identifier is reported only once
arch/s390/hypfs/hypfs_diag.c:384: error: for each function it appears in.)

Reported-by: Sachin Sant <sachinp@in.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>