Linux 4.16 compat: use correct *_dec_and_test()

Use refcount_dec_and_test() on 4.16+ kernels, atomic_dec_and_test()
on older kernels.  https://lwn.net/Articles/714974/

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes: #7179 
Closes: #7211

config/kernel-acl-refcount.m4

@@ -0,0 +1,20 @@
+dnl #
+dnl # 4.16 kernel: check if struct posix_acl acl.a_refcount is a refcount_t.
+dnl # It's an atomic_t on older kernels.
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_ACL_HAS_REFCOUNT], [
+	AC_MSG_CHECKING([whether posix_acl has refcount_t])
+	ZFS_LINUX_TRY_COMPILE([
+		#include <linux/backing-dev.h>
+		#include <linux/refcount.h>
+		#include <linux/posix_acl.h>
+	],[
+		struct posix_acl acl;
+		refcount_t *r __attribute__ ((unused)) = &acl.a_refcount;
+	],[
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_ACL_REFCOUNT, 1, [posix_acl has refcount_t])
+	],[
+		AC_MSG_RESULT(no)
+	])
+])

config/kernel.m4

@@ -124,6 +124,7 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
 	ZFS_AC_KERNEL_HAVE_GENERIC_SETXATTR
 	ZFS_AC_KERNEL_CURRENT_TIME
 	ZFS_AC_KERNEL_VM_NODE_STAT
+	ZFS_AC_KERNEL_ACL_HAS_REFCOUNT
 
 	AS_IF([test "$LINUX_OBJ" != "$LINUX"], [
 		KERNEL_MAKE="$KERNEL_MAKE O=$LINUX_OBJ"

include/linux/vfs_compat.h

@@ -288,9 +288,13 @@ zpl_posix_acl_release(struct posix_acl *acl)
 {
 	if ((acl == NULL) || (acl == ACL_NOT_CACHED))
 		return;
-
+#ifdef HAVE_ACL_REFCOUNT
+	if (refcount_dec_and_test(&acl->a_refcount))
+		zpl_posix_acl_release_impl(acl);
+#else
 	if (atomic_dec_and_test(&acl->a_refcount))
 		zpl_posix_acl_release_impl(acl);
+#endif
 }
 #endif /* HAVE_POSIX_ACL_RELEASE */