btrfs-progs: use clearer var names in is_ssd()

btrfs-progs: use clearer var names in is_ssd()

is_ssd() uses nondescript variable names; path - to what?
disk - it's a dev_t not a disk name, unlike dev, which is
a name not a dev_t!

Rename some vars to make things hopefully clearer:

 wholedisk - the name of the node for the entire disk
 devno - the dev_t of the device we're mkfs'ing
 sysfs_path - the path in sysfs we ultimately check

Signed-off-by: Eric Sandeen <sandeen@redhat.com>