Replace the checks for MNTK_UNMOUNTF with a macro that does the same thing.

This patch defines a macro that checks for MNTK_UNMOUNTF and replaces
explicit checks with this macro. It has no effect on semantics, but
prepares the code for a future patch where there will also be a
NFS specific flag for "forced dismount about to occur".

Suggested by: kib
MFC after: 2 weeks