fix report_mount_progress never calling set_progress_header

[avg-I]

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 a10d50f, 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)

Types of changes

• [x ] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• [x ] My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• [x ] All commit messages are properly formatted and contain Signed-off-by.