f2fs: fix to use kvfree instead of kzfree

As Jiqun Li reported in bugzilla:

https://bugzilla.kernel.org/show_bug.cgi?id=202747



System can panic due to using wrong allocate/free function pair
in xattr interface:
- use kvmalloc to allocate memory
- use kzfree to free memory

Let's fix to use kvfree instead of kzfree, BTW, we are safe to
get rid of kzfree, since there is no such confidential data stored
as xattr, we don't need to zero it before free memory.

Fixes: 5222595d ("f2fs: use kvmalloc, if kmalloc is failed")
Reported-by: Jiqun Li <jiqun.li@unisoc.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>