All parameters after @filepos of btrfs_alloc_ordered_extent() can be
replaced with btrfs_file_extent structure.

This patch does the cleanup, meanwhile some points to note:

- Move btrfs_file_extent structure to ordered-data.h
  The structure is needed by both btrfs_alloc_ordered_extent() and
  can_nocow_extent(), but since btrfs_inode.h includes
  ordered-data.h, so we need to move the structure to ordered-data.h.

- Move the special handling of NOCOW/PREALLOC into
  btrfs_alloc_ordered_extent()
  This is to allow btrfs_split_ordered_extent() to properly split them
  for DIO.
  For now just move the handling into btrfs_alloc_ordered_extent() to
  simplify the callers.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>