A few improvements to load_powerpc.c:

 - include integrity.h for the pr_fmt()
 - move all error reporting out of get_cert_list()
 - use ERR_PTR() to better preserve error detail
 - don't use pr_err() for missing keys
Reviewed-by: NMimi Zohar <zohar@linux.ibm.com>
Signed-off-by: NRussell Currey <ruscur@russell.cc>
Signed-off-by: NAndrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230210080401.345462-26-ajd@linux.ibm.com