erofs: fix missing unmap if z_erofs_get_extent_compressedlen() fails

Otherwise, meta buffers could be leaked.

Fixes: cec6e93b ("erofs: support parsing big pcluster compress indexes")
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20221205150050.47784-1-hsiangkao@linux.alibaba.com