lvmlocks: rework dm_strncpy

lvmlocks: rework dm_strncpy

Fix cutting away 1 character via incorrect usage of dm_strncpy
introduced in last batch of commits and use sizeof(buffer) to
get proper sizes.

In case of use strcpy_name_len() the replacement was invalid,
so we need to restore this case since sanlock uses buffer without
nul ending, so we would strip one more character from the buffer.

Also start to use dm_strncpy() without (void) for unchecked usage.