sys/vfs/hammer: Rename ondisk vol_name to vol_label

sys/vfs/hammer: Rename ondisk vol_name to vol_label

Ondisk volume header having a field named vol_name for a label
(not a block device path) is confusing, especially when inmemory
volume structure has vol_name for a block device but not a label.

There is even a kprintf message wrongly using ondisk vol_name
as a block device path, as well as some comments saying vol_name
is a filesystem label but not a path, which implies the name was
misleading.

This commit changes ondisk vol_name to vol_label.
This commit also changes vol_name in struct hammer_ioc_info to
vol_label. Outbox userspace programs using these two will see
compile error after this commit (which I doubt there is any...).
This commit doesn't break binaries.

Note that vol_name in struct libhammer_fsinfo is unchanged.