Re-share zfsdev_getminor and zfs_onexit_fd_hold

By adding a zfs_file_private accessor to the common
interfaces and some extensions to FreeBSD platform
code it is now possible to share the implementations
for the aforementioned functions.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #10073

include/sys/zfs_file.h

@@ -57,5 +57,6 @@ int zfs_file_unlink(const char *);
 
 int zfs_file_get(int fd, zfs_file_t **fp);
 void zfs_file_put(int fd);
+void *zfs_file_private(zfs_file_t *fp);
 
 #endif /* _SYS_ZFS_FILE_H */