nvram: fix "Segmentation fault" caused by setting memory out of buffer

Some MTD partitions with NVRAM have content starting in the middle. In
such case offset is set and nvram_header returns pointer to the middle.
It means we have to respect offset when calculating remaining space.

By the way use real MTD partition size (nvram_part_size variable) as we
may want to bump NVRAM_SPACE in the (very near) future.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46251 3c298f89-4303-0410-b956-a3cf2f4a3e73