Revert "f2fs: enhance multi-threads performance"

This reports performance regression by Yuanhan Liu.
The basic idea was to reduce one-point mutex, but it turns out this causes
another contention like context swithes.

https://lkml.org/lkml/2015/4/21/11

Until finishing the analysis on this issue, I'd like to revert this for a while.

This reverts commit 78373b7319abdf15050af5b1632c4c8b8b398f33.
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 160b883..b2dd1b0 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -1035,6 +1035,7 @@
 	sbi->raw_super = raw_super;
 	sbi->raw_super_buf = raw_super_buf;
 	mutex_init(&sbi->gc_mutex);
+	mutex_init(&sbi->writepages);
 	mutex_init(&sbi->cp_mutex);
 	init_rwsem(&sbi->node_write);
 	clear_sbi_flag(sbi, SBI_POR_DOING);