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>

fs/f2fs/file.c

@@ -4443,8 +4443,13 @@ static ssize_t f2fs_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
 		clear_inode_flag(inode, FI_NO_PREALLOC);
 
 		/* if we couldn't write data, we should deallocate blocks. */
-		if (preallocated && i_size_read(inode) < target_size)
+		if (preallocated && i_size_read(inode) < target_size) {
+			down_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
+			down_write(&F2FS_I(inode)->i_mmap_sem);
 			f2fs_truncate(inode);
+			up_write(&F2FS_I(inode)->i_mmap_sem);
+			up_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
+		}
 
 		if (ret > 0)
 			f2fs_update_iostat(F2FS_I_SB(inode), APP_WRITE_IO, ret);