f2fs: support norecovery mount option

This patch adds a mount option, norecovery, which is mostly same as
disable_roll_forward. The only difference is that norecovery should be activated
with read-only mount option.

This can be used when user wants to check whether f2fs is mountable or not
without any recovery process. (e.g., xfstests/200)

Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Documentation/filesystems/f2fs.txt

@@ -106,6 +106,8 @@ background_gc=%s       Turn on/off cleaning operations, namely garbage
                        Default value for this option is on. So garbage
                        collection is on by default.
 disable_roll_forward   Disable the roll-forward recovery routine
+norecovery             Disable the roll-forward recovery routine, mounted read-
+                       only (i.e., -o ro,disable_roll_forward)
 discard                Issue discard/TRIM commands when a segment is cleaned.
 no_heap                Disable heap-style segment allocation which finds free
                        segments for data from the beginning of main area, while