exec: Don't reset euid and egid when the tracee has CAP_SETUID

Don't reset euid and egid when the tracee has CAP_SETUID in
it's user namespace.  I punted on relaxing this permission check
long ago but now that I have read this code closely it is clear
it is safe to test against CAP_SETUID in the user namespace.
Signed-off-by: N"Eric W. Biederman" <ebiederm@xmission.com>

security/commoncap.c

@@ -550,7 +550,7 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
 	     !cap_issubset(new->cap_permitted, old->cap_permitted)) &&
 	    bprm->unsafe & ~LSM_UNSAFE_PTRACE_CAP) {
 		/* downgrade; they get no more than they had, and maybe less */
-		if (!capable(CAP_SETUID) ||
+		if (!ns_capable(new->user_ns, CAP_SETUID) ||
 		    (bprm->unsafe & LSM_UNSAFE_NO_NEW_PRIVS)) {
 			new->euid = new->uid;
 			new->egid = new->gid;