MIPS: bootmem: When relocatable, free memory below kernel

The kernel reserves all memory before the _end symbol as bootmem,
however, once the kernel can be relocated elsewhere in memory this may
result in a large amount of wasted memory. The assumption is that the
memory between the link and relocated address of the kernel may be
released back to the available memory pool.

Memory statistics for a Malta with the kernel relocating by
16Mb, without the patch:
Memory: 105952K/131072K available (4604K kernel code, 242K rwdata,
892K rodata, 1280K init, 183K bss, 25120K reserved, 0K cma-reserved)
And with the patch:
Memory: 122336K/131072K available (4604K kernel code, 242K rwdata,
892K rodata, 1280K init, 183K bss, 8736K reserved, 0K cma-reserved)

The 16Mb offset is removed from the reserved region and added back to
the available region.

Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Cc: Aaro Koskinen <aaro.koskinen@nokia.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Cc: Jaedon Shin <jaedon.shin@gmail.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: Jonas Gorski <jogo@openwrt.org>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: kernel-hardening@lists.openwall.com
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12986/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>