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>