erofs: clean up encoded map flags

- Remove EROFS_MAP_ENCODED since it was always set together with
  EROFS_MAP_MAPPED for compressed extents and checked redundantly;

- Replace the EROFS_MAP_FULL_MAPPED flag with the opposite
  EROFS_MAP_PARTIAL_MAPPED flag so that extents are implicitly
  fully mapped initially to simplify the logic;

- Make fragment extents independent of EROFS_MAP_MAPPED since
  they are not directly allocated on disk; thus fragment extents
  are no longer twisted with mapped extents.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>