ext2fs: Fix the size of struct ufid and add a static assert

File system specific *fid structures are copied into the generic
struct fid defined in sys/mount.h.
As such, they cannot be larger than struct fid.

This patch packed the structure and checks via a __Static_assert().

Reviewed by:	markj
MFC after:	2 weeks

(cherry picked from commit bfc8e3308b)

sys/fs/ext2fs/ext2_vnops.c

@@ -1884,6 +1884,8 @@ ext2_vptofh(struct vop_vptofh_args *ap)
 {
 	struct inode *ip;
 	struct ufid *ufhp;
+	_Static_assert(sizeof(struct ufid) <= sizeof(struct fid),
+	    "struct ufid cannot be larger than struct fid");
 
 	ip = VTOI(ap->a_vp);
 	ufhp = (struct ufid *)ap->a_fhp;

sys/fs/ext2fs/inode.h

@@ -189,7 +189,7 @@ struct ufid {
 	uint16_t ufid_pad;		/* Force 32-bit alignment. */
 	ino_t	ufid_ino;		/* File number (ino). */
 	uint32_t ufid_gen;		/* Generation number. */
-};
+} __packed;
 #endif	/* _KERNEL */
 
 #endif	/* !_FS_EXT2FS_INODE_H_ */