btrfs: use full subcommand name at _btrfs_get_subvolid()

btrfs: use full subcommand name at _btrfs_get_subvolid()

Avoid using the shortcut "sub" for the "subvolume" command, as this is the
standard practice because such shortcuts are not guaranteed to exist in
every btrfs-progs release (they may come and go). Also make the variables
local.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>