LinuxKPI: add dev_alloc_pages()

Add dev_alloc_pages() though it seems a weird KPI, not passing a dev
despite its name.  Used by updated wireless driver.

Sponsored by:	The FreeBSD Foundation
MFC after:	3 days
Reviewed by:	emaste
Differential Revision: https://reviews.freebsd.org/D44588

sys/compat/linuxkpi/common/include/linux/gfp.h

@@ -137,6 +137,12 @@ __free_page(struct page *page)
 	linux_free_pages(page, 0);
 }
 
+static inline struct page *
+dev_alloc_pages(unsigned int order)
+{
+	return (linux_alloc_pages(GFP_ATOMIC, order));
+}
+
 /*
  * Page management for mapped pages:
  */