ext4: fix type-widening bug in inode table readahead code

Due to a missing cast, the high 32-bits of a 64-bit block number used
when calculating the readahead block for inode tables can get lost.
This means we can end up fetching the wrong blocks for readahead for
file systems > 16TB.

Linus found this when experimenting with an enhacement to the sparse
static code checker which checks for missing widening casts before
binary "not" operators.
Reported-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/inode.c

@@ -4011,13 +4011,14 @@ static int __ext4_get_inode_loc(struct inode *inode,
 		if (EXT4_SB(sb)->s_inode_readahead_blks) {
 			ext4_fsblk_t b, end, table;
 			unsigned num;
+			__u32 ra_blks = EXT4_SB(sb)->s_inode_readahead_blks;
 
 			table = ext4_inode_table(sb, gdp);
 			/* s_inode_readahead_blks is always a power of 2 */
-			b = block & ~(EXT4_SB(sb)->s_inode_readahead_blks-1);
+			b = block & ~((ext4_fsblk_t) ra_blks - 1);
 			if (table > b)
 				b = table;
-			end = b + EXT4_SB(sb)->s_inode_readahead_blks;
+			end = b + ra_blks;
 			num = EXT4_INODES_PER_GROUP(sb);
 			if (ext4_has_group_desc_csum(sb))
 				num -= ext4_itable_unused_count(sb, gdp);