linuxkpi: Add bitmap_alloc and bitmap_free

This is a simple call to kmallock_array/kfree, therefore include linux/slab.h as
this is where the kmalloc_array/kfree definition is.

Sponsored-by: The FreeBSD Foundation
Reviewed by:	hselsasky
Differential Revision:	https://reviews.freebsd.org/D24794

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

@@ -30,6 +30,7 @@
 #define	_LINUX_BITMAP_H_
 
 #include <linux/bitops.h>
+#include <linux/slab.h>
 
 static inline void
 bitmap_zero(unsigned long *addr, const unsigned int size)
@@ -309,4 +310,10 @@ bitmap_xor(unsigned long *dst, const unsigned long *src1,
 		dst[i] = src1[i] ^ src2[i];
 }
 
+static inline void
+bitmap_free(const unsigned long *bitmap)
+{
+	kfree(bitmap);
+}
+
 #endif					/* _LINUX_BITMAP_H_ */