Fix loader bug causing too many pages allocation when bootloader is U-Boot

FreeBSD loader expects to have mmsz variable set by bootloader.
U-Boot behaviour is that if buffer size is not big enough to keep
whole memory map, assign the smallest correct buffer size to sz
and return error.

In other words U-Boot assumes that nobody will need mmsz value when buffer
is not filled with memory map, which is not true, so calculated pages value
was too big to allocate.

Solution: Simply assign default value to mmsz.

Submitted by: Patryk Duda <pdk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Semihalf
Differential Revision: https://reviews.freebsd.org/D12194

sys/boot/efi/loader/bootinfo.c

@@ -267,6 +267,13 @@ bi_load_efi_data(struct preloaded_file *kfp)
 
 	efisz = (sizeof(struct efi_map_header) + 0xf) & ~0xf;
 
+	/*
+	 * Assgin size of EFI_MEMORY_DESCRIPTOR to keep compatible with
+	 * u-boot which doesn't fill this value when buffer for memory
+	 * descriptors is too small (eg. 0 to obtain memory map size)
+	 */
+	mmsz = sizeof(EFI_MEMORY_DESCRIPTOR);
+
 	/*
 	 * It is possible that the first call to ExitBootServices may change
 	 * the map key. Fetch a new map key and retry ExitBootServices in that