xen/arm: kernel: Propagate the error if we fail to decompress the kernel

xen/arm: kernel: Propagate the error if we fail to decompress the kernel

Currently, we are ignoring any error from perform_gunzip() and replacing
the compressed kernel with the "uncompressed" kernel.

If there is a gzip failure, then it means that the output buffer may
contain garbagge. So it can result to various sort of behavior that may
be difficult to root cause.

In case of failure, free the output buffer and propagate the error.
We also need to adjust the return check for kernel_compress() as
perform_gunzip() may return a positive value.

Take the opportunity to adjust the code style for the check.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Michal Orzel <michal.orzel@arm.com>