ubifs: Import atomic_long operations from Linux

This commit is a preperation for a subsequent UBIFS commit
which needs atomic_long operations.

Therefor "include/asm-generic/atomic-long.h" is imported
from 1860e37 Linux 3.15

Signed-off-by: Anton Habegger <anton.habegger@gmail.com>
diff --git a/fs/ubifs/ubifs.c b/fs/ubifs/ubifs.c
index 49e6f46..6dd6174 100644
--- a/fs/ubifs/ubifs.c
+++ b/fs/ubifs/ubifs.c
@@ -150,6 +150,12 @@
 
 	return 0;
 }
+
+/* from shrinker.c */
+
+/* Global clean znode counter (for all mounted UBIFS instances) */
+atomic_long_t ubifs_clean_zn_cnt;
+
 #endif
 
 /**