Solaris: prevent AFS umount while busy

Solaris: prevent AFS umount while busy

Return EBUSY from unmount if someone still references stuff in AFS.
This prevents kernel panics that can occur on shutdown if we umount
while there is a file in AFS open. Normally a process can hold a file
in AFS open, AFS is unmounted, and the file is closed, triggering our
code which explodes if called after we're unmounted.

This adds VFS_HOLD/VFS_RELE calls whenever we 'create' a vcache, or
retire an old one, to keep track if anyone has an open reference to
us.

Change-Id: I95d8cf7e7e4d32a05bee97e06832a530b40af217
Reviewed-on: http://gerrit.openafs.org/1880
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>