kdump: crashkernel parameter improvement

Hi,

Here is a try to improve current crashkernel kernel parameter

Patch 1/3 adds an extra functionality so that one can use like crashkernel=2G-:128M^12 to reserve 128M for 2G+ machine but also scale the size based on system memory, that means 128M + (total_mem - 128M) >> 12

Patch 2/3 is a resending of previous post to let crashkernel=xM to reserve first from below 896M, then <4G, then <MAXMEM

Patch 3/3 round up total system memory size because according to test the mem size from memblock api is usually smaller than actual size, this passed my tests on x86/arm/powerpc, it should work for most cases, but there might be exceptions, appreciated for any better idea.

Thanks
Dave