zramctl: fix truncation warning

zramctl: fix truncation warning

sys-utils/zramctl.c: In function ‘zram_get_sysfs’:
sys-utils/zramctl.c:220:52: warning: ‘%s’ directive output may be truncated
writing up to 4095 bytes into a region of size 27 [-Wformat-truncation=]
     snprintf(z->devname, sizeof(z->devname), "/dev/%s", name);

As an additional good thing zramctl will no longer allocate 4096 bytes from
stack when just 23 bytes is enough.

[kzak@redhat.com: - use macro rather than hardcoded string for the path]

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
Signed-off-by: Karel Zak <kzak@redhat.com>