ext4lazyinit is a global thread. This thread performs itable
initalization under li_list_mtx mutex.

It basically does the following:
ext4_lazyinit_thread
  ->mutex_lock(&eli->li_list_mtx);
  ->ext4_run_li_request(elr)
    ->ext4_init_inode_table-> Do a lot of IO if the list is large

And when new mount/umount arrive they have to block on ->li_list_mtx
because  lazy_thread holds it during full walk procedure.
ext4_fill_super
 ->ext4_register_li_request
   ->mutex_lock(&ext4_li_info->li_list_mtx);
   ->list_add(&elr->lr_request, &ext4_li_info >li_request_list);
In my case mount takes 40minutes on server with 36 * 4Tb HDD.
Common user may face this in case of very slow dev ( /dev/mmcblkXXX)
Even more. If one of filesystems was frozen lazyinit_thread will simply
block on sb_start_write() so other mount/umount will be stuck forever.

This patch changes logic like follows:
- grab ->s_umount read sem before processing new li_request.
  After that it is safe to drop li_list_mtx because all callers of
  li_remove_request are holding ->s_umount for write.
- li_thread skips frozen SB's

Locking order:
Mh KOrder is asserted by umount path like follows: s_umount ->li_list_mtx so
the only way to to grab ->s_mount inside li_thread is via down_read_trylock

xfstests:ext4/023
#PSBM-49658
Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>