erofs: fold in z_erofs_decompress()

No need this helper since it's just a simple wrapper for decompress
method and only one caller.  So, let's fold in directly instead.

Signed-off-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230426084449.12781-1-zbestahu@gmail.com


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