tarfs_mount allow control of vfs_mountedfrom

[sjg]

Summary

We default to passing the path of the tar file to vfs_mountedfrom
so we can tell where a filesystem was mounted from.
However this can make the output of mount(8) hard to read.

Allow things like:

mount -t tarfs -o as=basename $tar $tar /mnt

so "as" is recorded instead of $tar

Sponsored by: Juniper Networks

[des]

sys/fs/tarfs/tarfs_vfsops.c

908

merge with next line

947

pull up

[sjg]

Tweak per feedback