ext4: add shutdown bit and check for it

Add a shutdown bit that will cause ext4 processing to fail immediately
with EIO.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/ext4.h

@@ -1836,6 +1836,12 @@ static inline bool ext4_has_incompat_features(struct super_block *sb)
  * Superblock flags
  */
 #define EXT4_FLAGS_RESIZING	0
+#define EXT4_FLAGS_SHUTDOWN	1
+
+static inline int ext4_forced_shutdown(struct ext4_sb_info *sbi)
+{
+	return test_bit(EXT4_FLAGS_SHUTDOWN, &sbi->s_ext4_flags);
+}
 
 
 /*