Avoid showing stale errors when nmount(2) fails.

Sometimes nmount(2) will fail without setting errmsg. The previous (ignored)
error would then be shown as the reason for the failed call if the next
nmount(2) also fails without [ENOENT,ENOTSUP].

An example is when there is a tmpfs mounted with -o size. vfs_filteropt() adds
'size' as an error in errmsg due to 'size' not being in tmpfs_updateopts. Then
tmpfs_mount returns [ENOTSUP] from nmount(2), which is then ignored. The next
call may race with an unmount causing an invalid [EINVAL] that then does log an
error, with the tmpfs errmsg.

The race itself is a separate issue to fix as it is expected to have an
[ENOENT] returned instead.

In this example the mount being shown is actually nullfs, not tmpfs that the
error is from.

  mountd[740]: can't delete exports for /poudriere/data/.m/exp-head-commit-test-devel/04/.npkg: Invalid argument mount option <size> is unknown

It should only show:

  mountd[740]: can't delete exports for /poudriere/data/.m/exp-head-commit-test-devel/04/.npkg: Invalid argument

MFC after:	2 weeks