f2fs: avoid double lock in truncate_blocks

The init_inode_metadata calls truncate_blocks when error is occurred.
The callers holds f2fs_lock_op, so we should not call it again in
truncate_blocks.

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

Conflicts:
	fs/f2fs/data.c

Change-Id: I8f9947c79c6f801f91dad23327f89519f58e1861