quota: split out netlink notification support from quota.c

Instead of adding ifdefs just split it into a new file.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>
diff --git a/fs/quota/Makefile b/fs/quota/Makefile
index 68d4f6d..dcba204 100644
--- a/fs/quota/Makefile
+++ b/fs/quota/Makefile
@@ -3,3 +3,4 @@
 obj-$(CONFIG_QFMT_V2)		+= quota_v2.o
 obj-$(CONFIG_QUOTA_TREE)	+= quota_tree.o
 obj-$(CONFIG_QUOTACTL)		+= quota.o
+obj-$(CONFIG_QUOTA_NETLINK_INTERFACE)	+= netlink.o