LinuxKPI: move __kmalloc from slab.h to slab.c

In order to allow the allocator to change in the future move it into
the implementation file from being an inline function in the header.

While here factor out the size calculation and add a comment as-to why
this is done.  We will need the size (_s) in the future to make a
decision on how to allocate.

Sponsored by:	The FreeBSD Foundation
MFC after:	3 days
Reviewed by:	emaste
Differential Revision: https://reviews.freebsd.org/D45815

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

@@ -92,6 +92,8 @@ struct linux_kmem_cache;
 #define ZERO_OR_NULL_PTR(x)	((x) == NULL || (x) == ZERO_SIZE_PTR)
 
 extern void *lkpi_kmalloc(size_t size, gfp_t flags);
+void *lkpi___kmalloc(size_t size, gfp_t flags);
+#define	__kmalloc(_s, _f)	lkpi___kmalloc(_s, _f)
 
 static inline gfp_t
 linux_check_m_flags(gfp_t flags)
@@ -108,13 +110,6 @@ linux_check_m_flags(gfp_t flags)
 	return (flags & GFP_NATIVE_MASK);
 }
 
-static inline void *
-__kmalloc(size_t size, gfp_t flags)
-{
-	return (malloc(MAX(size, sizeof(struct llist_node)), M_KMALLOC,
-	    linux_check_m_flags(flags)));
-}
-
 static inline void *
 kmalloc_node(size_t size, gfp_t flags, int node)
 {

sys/compat/linuxkpi/common/src/linux_slab.c

@@ -207,6 +207,17 @@ linux_kmem_cache_destroy(struct linux_kmem_cache *c)
 	free(c, M_KMALLOC);
 }
 
+void *
+lkpi___kmalloc(size_t size, gfp_t flags)
+{
+	size_t _s;
+
+	/* sizeof(struct llist_node) is used for kfree_async(). */
+	_s = MAX(size, sizeof(struct llist_node));
+
+	return (malloc(_s, M_KMALLOC, linux_check_m_flags(flags)));
+}
+
 struct lkpi_kmalloc_ctx {
 	size_t size;
 	gfp_t flags;