Fix report_mount_progress never calling set_progress_header

That happens because of an off-by-one mistake.
share_mount_one_cb() calls report_mount_progress(current=sm_done) after
having incremented sm_done by one. Then report_mount_progress()
increments the parameter again. It appears that that logic became
obsolete after commit a10d50f999511, parallel zfs mount.

On FreeBSD I observe that zfs mount -a -v prints, for example,

(null): (201/248)

That happens because set_progress_header() is never called.

With this change the output becomes correct:

    Mounting ZFS filesystems: (209/248)

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Andriy Gapon <avg@FreeBSD.org>
Closes #11607