f2fs: Fix deadlock between f2fs_quota_sync and block_operation

This deadlock is hitting Android users (Pixel 3/3a/4) with Magisk, due
to frequent umount/mount operations that trigger quota_sync, hitting
the race. See https://github.com/topjohnwu/Magisk/issues/3171

 for
additional impact discussion.

In commit db6ec53b7e03, we added a semaphore to protect quota flags.
As part of this commit, we changed f2fs_quota_sync to call
f2fs_lock_op, in an attempt to prevent an AB/BA type deadlock with
quota_sem locking in block_operation.  However, rwsem in Linux is not
recursive. Therefore, the following deadlock can occur:

f2fs_quota_sync
down_read(cp_rwsem) // f2fs_lock_op
filemap_fdatawrite
f2fs_write_data_pages
...
                                   block_opertaion
				   down_write(cp_rwsem) - marks rwsem as
				                          "writer pending"
down_read_trylock(cp_rwsem) - fails as there is
                              a writer pending.
			      Code keeps on trying,
			      live-locking the filesystem.

We solve this by creating a new rwsem, used specifically to
synchronize this case, instead of attempting to reuse an existing
lock.

Signed-off-by: Shachar Raindel <shacharr@gmail.com>

Fixes: db6ec53b7e03 f2fs: add a rw_sem to cover quota flag changes
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
Signed-off-by: iusmac <iusico.maxim@libero.it>
Signed-off-by: improbir <ghosh_probir@yahoo.com>