drivers/net/mlx4/alloc.c needs mm.h

sparc32 allmodconfig with linux-next:

drivers/net/mlx4/alloc.c: In function 'mlx4_buf_alloc':
drivers/net/mlx4/alloc.c:164: error: 'PAGE_KERNEL' undeclared (first use in this function)
drivers/net/mlx4/alloc.c:164: error: (Each undeclared identifier is reported only once
drivers/net/mlx4/alloc.c:164: error: for each function it appears in.)

this is due to some header shuffle in linux-next.  I didn't look to see what
it was.  I'd sugges that this patch be merged ahead of a linux-next merge to
avoid bisection breaks.

We strictly only need asm/pgtable.h, but going direct to asm includes always
seems grubby.

Cc: Jeff Garzik <jeff@garzik.org>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>