ext4: fix oops when loading block bitmap failed

When we fail to load block bitmap in __ext4_new_inode() we will
dereference NULL pointer in ext4_journal_get_write_access(). So check
for error from ext4_read_block_bitmap().

Coverity-id: 989065
Cc: stable@vger.kernel.org
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/ialloc.c

@@ -887,6 +887,10 @@ got:
 		struct buffer_head *block_bitmap_bh;
 
 		block_bitmap_bh = ext4_read_block_bitmap(sb, group);
+		if (!block_bitmap_bh) {
+			err = -EIO;
+			goto out;
+		}
 		BUFFER_TRACE(block_bitmap_bh, "get block bitmap access");
 		err = ext4_journal_get_write_access(handle, block_bitmap_bh);
 		if (err) {