btrfs-progs: use proper size for argv0 substitution

Make run from a long base path will overflow the argv0 buffer during
tests. Otherwise, this would happen for all the standalone binaries that
use set_argv0.

Original report:
https://bbs.archlinux.org/viewtopic.php?id=189861

Reported-by: WorMzy Tykashi <wormzy.tykashi@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
btrfs-progs: fix compiler warning

gcc 4.9.0 gives a warning: array subscript is above array bounds

Checking for "greater or equal" instead of just "equal" fixes this.

The warning is a false positive, appears with -ftree-vrp, but we'd
rather fix it to avoid noise during build.

http://www.mail-archive.com/linux-btrfs%40vger.kernel.org/msg34338.html

Signed-off-by: Christian Hesse <mail@eworm.de>
Signed-off-by: David Sterba <dsterba@suse.cz>