ext4: init timer earlier to avoid a kernel panic in __save_error_info

During mount, when we fail to open journal inode or root inode, the
__save_error_info will mod_timer. But actually s_err_report isn't
initialized yet and the kernel oops. The detailed information can
be found https://bugzilla.kernel.org/show_bug.cgi?id=32082.

The best way is to check whether the timer s_err_report is initialized
or not. But it seems that in include/linux/timer.h, we can't find a
good function to check the status of this timer, so this patch just
move the initializtion of s_err_report earlier so that we can avoid
the kernel panic. The corresponding del_timer is also added in the
error path.

Reported-by: Sami Liedes <sliedes@cc.hut.fi>
Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>