attempting to remove nonexistant exports with MNT_DELEXPORT returns
an error; before this change it always succeeded. This caused
mountd(8) to log "can't delete exports for /whatever" warnings.
Change the error code from EINVAL to a more specific ENOENT, and
make mountd ignore this error when deleting the export list. I
could have just restored the previous behaviour of returning success,
but I think an error return is a useful diagnostic.
Reviewed by: phk
5f558fa42f