f2fs: support F2FS_IOC_FS{GET,SET}XATTR

This patch adds FS_IOC_FSSETXATTR/FS_IOC_FSGETXATTR ioctl interface
support for f2fs. The interface is kept consistent with the one
of ext4/xfs.

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