btrfs-progs: utils: make sure set_label_mounted uses correct length buffers

btrfs-progs: utils: make sure set_label_mounted uses correct length buffers

When `btrfs filesystem label /foo bar` command is invoked, it will pass
the buffer allocated in the argv array directly to set_label_mounted()
and then to the BTRFS_IOC_SET_FSLABEL ioctl.

However, the kernel code handling the ioctl will always try to copy
BTRFS_LABEL_SIZE bytes[1] from the userland pointer. Under certain
conditions and when the label is small enough, the command will fail
with:

    [root@localhost /]# btrfs filesystem label /mnt f
    ERROR: unable to set label Bad address

Fix this by making sure we pass a BTRFS_LABEL_SIZE sized buffer to the
ioctl containing the desired label.

[1] https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/fs/btrfs/ioctl.c?id=refs/tags/v4.5#n5231

Signed-off-by: Petros Angelatos <petrosagg@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>