f2fs: protect new segment allocation in expand_inode_data

Found a new segemnt allocation without f2fs_lock_op() in
expand_inode_data(). So, when we do fallocate() for a pinned file
and trigger checkpoint very frequently and simultaneously. F2FS gets
stuck in the below code of do_checkpoint() forever.

  f2fs_sync_meta_pages(sbi, META, LONG_MAX, FS_CP_META_IO);
  /* Wait for all dirty meta pages to be submitted for IO */
                                                <= if fallocate() here,
  f2fs_wait_on_all_pages(sbi, F2FS_DIRTY_META); <= it'll wait forever.

Signed-off-by: Daeho Jeong <daehojeong@google.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>