x86/boot/compressed/64: Check return value of kernel_ident_mapping_init()

The function can fail to create an identity mapping, check for that
and bail out if it happens.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200907131613.12703-22-joro@8bytes.org