f2fs: get io size bit from mount option

This patch adds to set io_size_bits from mount option.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Documentation/filesystems/f2fs.txt

@@ -157,6 +157,8 @@ data_flush             Enable data flushing before checkpoint in order to
 mode=%s                Control block allocation mode which supports "adaptive"
                        and "lfs". In "lfs" mode, there should be no random
                        writes towards main area.
+io_bits=%u             Set the bit size of write IO requests. It should be set
+                       with "mode=lfs".
 
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