f2fs: add mount option for segment allocation policy

This patch adds an mount option, "alloc_mode=%s" having two options, "default"
and "reuse".

In "alloc_mode=reuse" case, f2fs starts to allocate segments from 0'th segment
all the time to reassign segments. It'd be useful for small-sized eMMC parts.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/Documentation/filesystems/f2fs.txt b/Documentation/filesystems/f2fs.txt
index e7fb61e..58a6783 100644
--- a/Documentation/filesystems/f2fs.txt
+++ b/Documentation/filesystems/f2fs.txt
@@ -171,6 +171,14 @@
 offprjjquota           Turn off project journelled quota.
 quota                  Enable plain user disk quota accounting.
 noquota                Disable all plain disk quota option.
+whint_mode=%s          Control which write hints are passed down to block
+                       layer. This supports "off", "user-based", and
+                       "fs-based".  In "off" mode (default), f2fs does not pass
+                       down hints. In "user-based" mode, f2fs tries to pass
+                       down hints given by users. And in "fs-based" mode, f2fs
+                       passes down hints with its policy.
+alloc_mode=%s          Adjust block allocation policy, which supports "reuse"
+                       and "default".
 
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