tmpfs tests: check for built-in tmpfs module

As of r363471, tmpfs is included in all GENERIC kernel configs. This
results in a warning being emitted for each call to kldload(8):

module_register: cannot register tmpfs from tmpfs.ko; already loaded from kernel

Check for the presence of the module via kldstat first to quiet this
warning.

Reviewed by:	asomers, arichardson
Differential Revision:	https://reviews.freebsd.org/D26632

contrib/netbsd-tests/fs/h_funcs.subr

@@ -45,7 +45,7 @@ require_fs() {
 
 	# Begin FreeBSD
 	if true; then
-		if kldload -n ${name}; then
+		if kldstat -qm ${name} || kldload -n ${name}; then
 			found=yes
 		else
 			found=no