Revert "f2fs: block cache/dio write during f2fs_enable_checkpoint()"

Revert "f2fs: block cache/dio write during f2fs_enable_checkpoint()"

This reverts commit 196c81fdd438f7ac429d5639090a9816abb9760a.

Original patch may cause below deadlock, revert it.

write				remount
- write_begin
 - lock_page  --- lock A
 - prepare_write_begin
  - f2fs_map_lock
				- f2fs_enable_checkpoint
				 - down_write(cp_enable_rwsem)  --- lock B
				 - sync_inode_sb
				  - writepages
				   - lock_page			--- lock A
   - down_read(cp_enable_rwsem)  --- lock A

Cc: stable@kernel.org
Fixes: 196c81fdd438 ("f2fs: block cache/dio write during f2fs_enable_checkpoint()")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>