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>

Documentation/filesystems/f2fs.txt

@@ -180,6 +180,8 @@ whint_mode=%s          Control which write hints are passed down to block
                        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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