Convert fsidcmp(9) from macro to inline function

This allows type checking the arguments.

Explicit structure members comparisions are done to avoid introducting
string.h pollution for userspace.

Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D43205

sys/sys/mount.h

@@ -50,7 +50,12 @@
 
 typedef struct fsid { int32_t val[2]; } fsid_t;	/* filesystem id type */
 
-#define fsidcmp(a, b) memcmp((a), (b), sizeof(fsid_t))
+/* Returns non-zero if fsids are different. */
+static inline int
+fsidcmp(const fsid_t *a, const fsid_t *b)
+{
+	return (a->val[0] != b->val[0] || a->val[1] != b->val[1]);
+}
 
 /*
  * File identifier.