erofs: improve decompression error reporting

erofs: improve decompression error reporting

Change the return type of decompress() from `int` to `const char *` to
provide more informative error diagnostics:

 - A NULL return indicates successful decompression;

 - If IS_ERR(ptr) is true, the return value encodes a standard negative
   errno (e.g., -ENOMEM, -EOPNOTSUPP) identifying the specific error;

 - Otherwise, a non-NULL return points to a human-readable error string,
   and the corresponding error code should be treated as -EFSCORRUPTED.

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