f2fs: fix to avoid race during access gc_thread pointer

Thread A			Thread B
- f2fs_remount
 - stop_gc_thread
				- f2fs_sbi_store
   sbi->gc_thread = NULL;
				  access sbi->gc_thread->gc_*

Previously, we allocate memory for sbi->gc_thread based on background
gc thread mount option, the memory can be released if we turn off
that mount option, but still there are several places access gc_thread
pointer without considering race condition, result in NULL point
dereference.

In order to fix this issue, use sb->s_umount to exclude those operations.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>