f2fs crypto: allow setting encryption policy once

This patch add XATTR_CREATE flag in setxattr when setting encryption
context for inode. Without this flag the context could be set more than
once, this should never happen. So, fix it.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>