__vfs_removexattr() transfers "NULL" value to the setxattr handler of
the f2fs filesystem in order to remove the extended attribute. But,
__f2fs_setxattr() just ignores the removal request when the value of
the extended attribute is already NULL. We have to remove the extended
attribute itself even if the value of that is already NULL.

We can reporduce this bug with the below:

1. touch file
2. setfattr -n "user.foo" file
3. setfattr -x "user.foo" file
4. getfattr -d file
> user.foo
Signed-off-by: NDaeho Jeong <daeho.jeong@samsung.com>
Signed-off-by: NYoungjin Gil <youngjin.gil@samsung.com>
Tested-by: NHobin Woo <hobin.woo@samsung.com>
Tested-by: NChao Yu <yuchao0@huawei.com>
Reviewed-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>