f2fs: fix to avoid GC/mmap race with f2fs_truncate()

It missed to hold i_gc_rwsem and i_map_sem around f2fs_truncate()
in f2fs_file_write_iter() to avoid racing with background GC and
mmap, fix it.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>