nvram: bogus partition listing

[nathanlynch]

Since a6d31ca, nvram --partitions with large enough argument passed to --nvram-size prints a bogus all-zeros entry at the end:

[root@ltc-zz14-lp1 ~]# ~test/powerpc-utils/src/nvram --partitions
 # Sig Chk  Len  Name
 0  50  ef  0024 of-config
 1  70  99  009d common
 2  a0  b5  0083 ibm,rtas-log
 3  a0  4f  00fb lnx,oops-log
 4  7f  9b  0181 wwwwwwwwwwww
[root@ltc-zz14-lp1 ~]# ~test/powerpc-utils/src/nvram --partitions --nvram-size $((16 * 1024))
/home/test/powerpc-utils/src/nvram: WARNING: expected 16384 bytes, but only read 15360!
 # Sig Chk  Len  Name
 0  50  ef  0024 of-config
 1  70  99  009d common
 2  a0  b5  0083 ibm,rtas-log
 3  a0  4f  00fb lnx,oops-log
 4  7f  9b  0181 wwwwwwwwwwww
 5  00  00  0000                        <<<<<<<<<<

When the user forces there to be excess space in the nvram buffer, nvram_parse_partitions() constructs a bogus entry from the (fortunately zeroed) remainder.

I propose either:

1. Ignoring --nvram-size with a warning, removing all supporting code. I don't know of a use for it, at least not on LPARs. Or:
2. Clamping the nvram buffer allocation to the lesser of the user-supplied value and the size reported by the device.