jbd,jbd2: fix oops in jbd2_journal_put_journal_head()

Commit ae4647fb (jbd2: reduce journal_head size) introduced a
regression where we occasionally hit panic in
jbd2_journal_put_journal_head() because of wrong b_jcount. The bug is
caused by gcc making 64-bit access to 32-bit bitfield and thus
clobbering b_jcount.

At least for now, those 8 bytes saved in struct journal_head are not
worth the trouble with gcc bitfield handling so revert that part of
the patch.

Reported-by: EUNBONG SONG <eunb.song@samsung.com>
Reported-by: Tony Luck <tony.luck@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

include/linux/journal-head.h

@@ -30,15 +30,19 @@ struct journal_head {
 
 	/*
 	 * Journalling list for this buffer [jbd_lock_bh_state()]
+	 * NOTE: We *cannot* combine this with b_modified into a bitfield
+	 * as gcc would then (which the C standard allows but which is
+	 * very unuseful) make 64-bit accesses to the bitfield and clobber
+	 * b_jcount if its update races with bitfield modification.
 	 */
-	unsigned b_jlist:4;
+	unsigned b_jlist;
 
 	/*
 	 * This flag signals the buffer has been modified by
 	 * the currently running transaction
 	 * [jbd_lock_bh_state()]
 	 */
-	unsigned b_modified:1;
+	unsigned b_modified;
 
 	/*
 	 * Copy of the buffer data frozen for writing to the log.