linuxkpi: Add atomic64_fetch_add

Linux variant of atomic_fetchadd_64.

Reviewed by:	bz
MFC after:	2 weeks
Sponsored by:	Beckhoff Automation GmbH & Co. KG
Differential Revision:	https://reviews.freebsd.org/D34296

(cherry picked from commit 1889bed)

sys/compat/linuxkpi/common/include/asm/atomic64.h

@@ -52,6 +52,12 @@ typedef struct {
 #define	atomic64_dec_return(v)		atomic64_sub_return(1, (v))
 #define	atomic64_inc_not_zero(v)	atomic64_add_unless((v), 1, 0)
 
+static inline int64_t
+atomic64_fetch_add(int64_t i, atomic64_t *v)
+{
+	return (atomic_fetchadd_64(&v->counter, i));
+}
+
 static inline int64_t
 atomic64_add_return(int64_t i, atomic64_t *v)
 {