ext4: add ext4_group_t, and change all group variables to this type.

In many places variables for block group are of type int, which limits the
maximum number of block groups to 2^31.  Each block group can have up to
2^15 blocks, with a 4K block size,  and the max filesystem size is limited to
2^31 * (2^15 * 2^12) = 2^58  -- or 256 PB

This patch introduces a new type ext4_group_t, of type unsigned long, to
represent block group numbers in ext4.
All occurrences of block group variables are converted to type ext4_group_t.

Signed-off-by: Avantika Mathur <mathur@us.ibm.com>
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 488f829..1ee19c9 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2464,7 +2464,8 @@
 static ext4_fsblk_t ext4_get_inode_block(struct super_block *sb,
 		unsigned long ino, struct ext4_iloc *iloc)
 {
-	unsigned long desc, group_desc, block_group;
+	unsigned long desc, group_desc;
+	ext4_group_t block_group;
 	unsigned long offset;
 	ext4_fsblk_t block;
 	struct buffer_head *bh;
@@ -2551,7 +2552,7 @@
 			struct ext4_group_desc *desc;
 			int inodes_per_buffer;
 			int inode_offset, i;
-			int block_group;
+			ext4_group_t block_group;
 			int start;
 
 			block_group = (inode->i_ino - 1) /