libselinux: make selinux_status_open(3) reentrant

Do not mmap the status page again if `selinux_status_open(3)` has already
been called with success.

`selinux_status_open(3)` might be called unintentionally multiple times,
e.g. once to manually be able to call `selinux_status_getenforce(3)` and
once indirectly through `selinux_check_access(3)`
(since libselinux 3.2).

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>

libselinux/src/sestatus.c

@@ -282,6 +282,10 @@ int selinux_status_open(int fallback)
 	long		pagesize;
 	uint32_t	seqno;
 
+	if (selinux_status != NULL) {
+		return 0;
+	}
+
 	if (!selinux_mnt) {
 		errno = ENOENT;
 		return -1;