shmfd: make shm_size a vm_ooffset_t

On 32-bit platforms, this expands the shm_size to a 64-bit quantity and
resolves a mismatch between the shmfd size and underlying vm_object size.
The implementation did not account for this kind of mismatch.

Reviewed by:	kib
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D25602

sys/sys/mman.h

@@ -254,7 +254,7 @@ typedef	__size_t	size_t;
 struct file;
 
 struct shmfd {
-	size_t		shm_size;
+	vm_ooffset_t	shm_size;
 	vm_object_t	shm_object;
 	int		shm_refs;
 	uid_t		shm_uid;