Fix getfsstat(2) with MNT_WAIT to not skip filesystems that are in the
process of being unmounted. Previously it would skip them, even if the
unmount eventually failed eg due to the filesystem being busy.

This behaviour broke autounmountd(8) - if you tried to manually unmount
a mounted filesystem, using 'automount -u', and the autounmountd attempted
to refresh the filesystem list in that very moment, it would conclude that
the filesystem got unmounted and not try to unmount it afterwards.

Reviewed by: kib@
Tested by: pho@
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D8030