selinux: remove unnecessary selinux cred request

task_security_struct was obtained at the beginning of may_create
and selinux_inode_init_security, no need to obtain again.
may_create will be called very frequently when create dir and file.

Cc: Paul Moore <paul@paul-moore.com>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: Eric Paris <eparis@parisplace.org>
Signed-off-by: Yang Guo <guoyang2@huawei.com>
Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>