Fix strdup conflict on other platforms

In the FreeBSD kernel the strdup signature is:

```
char	*strdup(const char *__restrict, struct malloc_type *);
```

It's unfortunate that the developers have chosen to change
the signature of libc functions - but it's what I have to
deal with.

Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9433

module/os/linux/spl/spl-kmem.c

@@ -120,18 +120,18 @@ __strdup(const char *str, int flags)
 }
 
 char *
-strdup(const char *str)
+kmem_strdup(const char *str)
 {
 	return (__strdup(str, KM_SLEEP));
 }
-EXPORT_SYMBOL(strdup);
+EXPORT_SYMBOL(kmem_strdup);
 
 void
-strfree(char *str)
+kmem_strfree(char *str)
 {
 	kfree(str);
 }
-EXPORT_SYMBOL(strfree);
+EXPORT_SYMBOL(kmem_strfree);
 
 /*
  * General purpose unified implementation of kmem_alloc(). It is an