ext4: use ext4_grpblk_t more extensively

unsigned  short is potentially too small to track blocks within
a group; today it is safe due to restrictions in e2fsprogs but
we have _lo / _hi bits for group blocks with the intent to go
up to 32 bits, so clean this up now.

There are many more places where we use unsigned/int/unsigned int
to contain a group block but this should at least fix all the
short types.

I added a few comments to the struct ext4_group_info definition
as well.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/fs/ext4/mballoc.h b/fs/ext4/mballoc.h
index 9db890d..188d3d7 100644
--- a/fs/ext4/mballoc.h
+++ b/fs/ext4/mballoc.h
@@ -136,8 +136,8 @@
 	unsigned		pa_deleted;
 	ext4_fsblk_t		pa_pstart;	/* phys. block */
 	ext4_lblk_t		pa_lstart;	/* log. block */
-	unsigned short		pa_len;		/* len of preallocated chunk */
-	unsigned short		pa_free;	/* how many blocks are free */
+	ext4_grpblk_t		pa_len;		/* len of preallocated chunk */
+	ext4_grpblk_t		pa_free;	/* how many blocks are free */
 	unsigned short		pa_type;	/* pa type. inode or group */
 	spinlock_t		*pa_obj_lock;
 	struct inode		*pa_inode;	/* hack, for history only */
@@ -152,7 +152,7 @@
 	ext4_lblk_t fe_logical;
 	ext4_grpblk_t fe_start;
 	ext4_group_t fe_group;
-	int fe_len;
+	ext4_grpblk_t fe_len;
 };
 
 /*