efi/libstub: Make efi_random_alloc() allocate below 4 GB on 32-bit

The UEFI stub executes in the context of the firmware, which identity
maps the available system RAM, which implies that only memory below
4 GB can be used for allocations on 32-bit architectures, even on [L]PAE
capable hardware.

So ignore any reported memory above 4 GB in efi_random_alloc(). This
also fixes a reported build problem on ARM under -Os, where the 64-bit
logical shift relies on a software routine that the ARM decompressor does
not provide.

A second [minor] issue is also fixed, where the '+ 1' is moved out of
the shift, where it belongs: the reason for its presence is that a
memory region where start == end should count as a single slot, given
that 'end' takes the desired size and alignment of the allocation into
account.

To clarify the code in this regard, rename start/end to 'first_slot' and
'last_slot', respectively, and introduce 'region_end' to describe the
last usable address of the current region.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/1480010543-25709-2-git-send-email-ard.biesheuvel@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>