SELinux: Use GFP_KERNEL for selinux_parse_opts_str().

Since selinux_parse_opts_str() is calling match_strdup() which uses
GFP_KERNEL, it is safe to use GFP_KERNEL from kcalloc() which is
called by selinux_parse_opts_str().

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Paul Moore <paul@paul-moore.com>