A refinement of change 232351 to avoid a race with a forcible unmount.

A refinement of change 232351 to avoid a race with a forcible unmount.
While we have a snapshot vnode unlocked to avoid a deadlock with another
inode in the same inode block being updated, the filesystem containing
it may be forcibly unmounted. When that happens the snapshot vnode is
revoked. We need to check for that condition and fail appropriately.

This change will be included along with 232351 when it is MFC'ed to 9.

Spotted by:  kib
Reviewed by: kib