Fix build tags for isRealProc function (prometheus#560)

Only aix, darwin, dragonfly freebsd and linux GOOS have a Type member in
their syscall.Statfs_t struct. However, since the isRealProc compares
that Type to a magic number (PROC_SUPER_MAGIC, 0x9fa0) which is only
relevant to Linux (and possibly FreeBSD with Linux compatibility),
adjust build tags accordingly.

This drops the previous nostatfs build tag workaround, since the
"tamago" GOOS for which it was originally added won't match the updated
build tags.

Fixes: prometheus#554.

Signed-off-by: Daniel Swarbrick <daniel.swarbrick@gmail.com>

fs_statfs_notype.go

@@ -11,8 +11,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-//go:build netbsd || openbsd || solaris || windows || nostatfs
-// +build netbsd openbsd solaris windows nostatfs
+//go:build !freebsd && !linux
+// +build !freebsd,!linux
 
 package procfs
 

fs_statfs_type.go

@@ -11,8 +11,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-//go:build !netbsd && !openbsd && !solaris && !windows && !nostatfs
-// +build !netbsd,!openbsd,!solaris,!windows,!nostatfs
+//go:build freebsd || linux
+// +build freebsd linux
 
 package procfs