afs: Remove 'slept' from osi_VM_FlushVCache

No implementation of osi_VM_FlushVCache drops and reacquires
afs_xvcache. Doing so would cause problems when afs_FlushVCache calls
osi_VM_FlushVCache, since someone could grab a reference to the vcache
while xvcache is dropped. So, prohibit dropping and reacquiring
afs_xvcache in osi_VM_FlushVCache, and remove the 'slept' argument to
it.

Change-Id: I50b4ee35f54a5277749f44e93b1094e4fb5c93e9
Reviewed-on: http://gerrit.openafs.org/7435
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
afs: Add xvcache-related afs_FlushVCache comments

Add a couple of comments to help make it explicit when it is okay to
drop afs_xvcache here.

Change-Id: I451ffe80755ea471322e32017f71c0619e6a8e63
Reviewed-on: http://gerrit.openafs.org/7436
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>