btrfs-progs: simplify ioctl name copy and null termination
In the places where we copy a string into the name
member of btrfs_ioctl_vol_args or btrfs_ioctl_vol_args_v2,
we use strncopy (to not overflow the name array) and then
set the last position to the null character.
Howver, in both cases the arrays are defined with:
char name[MAX+1];
hence the last array position is name[MAX].
In most cases, we now insert the null at name[MAX-1]
which deprives us of one useful character.
Even the above isn't consistent through the code, so
make some helper code to make it simple, i.e.
strncpy_null(dest, src) which automatically does the
right thing based on the size of dest.
Thanks to Zach Brown for the macro suggestion.
Signed-off-by: Eric Sandeen <sandeen@redhat.com>