erofs: remove need_kmap in erofs_read_metabuf()

 - need_kmap is always true except for a ztailpacking case; thus, just
   open-code that one;

 - The upcoming metadata compression will add a new boolean, so simplify
   this first.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20250714090907.4095645-1-hsiangkao@linux.alibaba.com