Just supped (in /usr/src make includes, in compile dir make cleandir && make
depend && make) and get:

../../../../kern/vfs_syscalls.c: In function `dounmount':
../../../../kern/vfs_syscalls.c:481: structure has no member named `mnt_wcnt'
../../../../kern/vfs_syscalls.c:483: structure has no member named `mnt_wcnt'
../../../../kern/vfs_syscalls.c:495: `MNT_GONE' undeclared (first use in this function)
../../../../kern/vfs_syscalls.c:497: structure has no member named `mnt_wcnt'
../../../../kern/vfs_syscalls.c:499: structure has no member named `mnt_wcnt'


From src/sys/kern/vfs_syscalls.c:dounmount()

481:            while(mp->mnt_wcnt > 0) {
                        wakeup((caddr_t)mp);
                        sleep(&mp->mnt_wcnt, PVFS);
                }

497:    while(mp->mnt_wcnt > 0) {
                wakeup((caddr_t)mp);
                sleep(&mp->mnt_wcnt, PVFS);
        }

From /usr/include/sys/mount.h:

struct mount {
        CIRCLEQ_ENTRY(mount) mnt_list;          /* mount list */
        struct vfsops   *mnt_op;                /* operations on fs */
        struct vnode    *mnt_vnodecovered;      /* vnode we mounted on */
        struct vnodelst mnt_vnodelist;          /* list of vnodes this mount */
        struct lock     mnt_lock;               /* mount structure lock */
        int             mnt_flag;               /* flags */
        int             mnt_maxsymlinklen;      /* max size of short symlink */
        struct statfs   mnt_stat;               /* cache of filesystem stats */
        qaddr_t         mnt_data;               /* private data */
};

?

Cheers,

Patrick