efi-bgrt: Add error handling; inform the user when ignoring the BGRT

efi-bgrt: Add error handling; inform the user when ignoring the BGRT

Gracefully handle failures to allocate memory for the image, which might
be arbitrarily large.

efi_bgrt_init can fail in various ways as well, usually because the
BIOS-provided BGRT structure does not match expectations.  Add
appropriate error messages rather than failing silently.

Reported-by: Srihari Vijayaraghavan <[email protected]>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=81321
Signed-off-by: Josh Triplett <[email protected]>
Signed-off-by: Matt Fleming <[email protected]>