Btrfs-progs: specify label length larger than 255 bytes cause mkfs.btrfs buffer overflow

Btrfs-progs: specify label length larger than 255 bytes cause mkfs.btrfs buffer overflow

Hello,

While going through the mkfs.c, I noticed there is an issue for label
length checking, mkfs.btrfs will crashed if the label length exceeding
255 bytes, it's easy to triggered that out as below:

jeff@pibroch:~/opensource/btrfs-progs$ sudo ./mkfs.btrfs -L `perl -e
'print "A"x256'` /usr/src/linux-3.0/img0

WARNING! - Btrfs v0.19-35-g1b444cd IS EXPERIMENTAL
WARNING! - see http://btrfs.wiki.kernel.org before using

*** buffer overflow detected ***: ./mkfs.btrfs terminated
======= Backtrace: =========
/lib/i386-linux-gnu/libc.so.6(__fortify_fail+0x50)[0xb7774df0]
/lib/i386-linux-gnu/libc.so.6(+0xe4cca)[0xb7773cca]
/lib/i386-linux-gnu/libc.so.6(__strcpy_chk+0x3f)[0xb777305f]
./mkfs.btrfs[0x805acc4]
./mkfs.btrfs[0x805def6]
/lib/i386-linux-gnu/libc.so.6(__libc_start_main+0xe7)[0xb76a5e37]
./mkfs.btrfs[0x8048ef1]
======= Memory map: ========
......

a tiny patch could fix it.

Signed-off-by: Jie Liu <jeff.liu@oracle.com>