fsck.f2fs: support a preen mode
fsck.f2fs: support a preen mode

Generic fsck uses -p, preen mode, and fsck.f2fs can support it as -a for now.

Change-Id: Ib3b31d10feb8f778540377424f91fb0e5e690cc0
Signed-off-by: Jaegeuk Kim <71255387a32c360dfbe6dfb7aaeb123bf1dc8cb6@kernel.org>

Yu-AndroidM/android_external_f2fs-tools,Yu-AndroidM/android_external_f2fs-tools