f2fs: introduce f2fs_encrypted_file for clean-up

This patch replaces (f2fs_encrypted_inode() && S_ISREG()) with
f2fs_encrypted_file(), which gives no functional change.

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