client: pin Inode during readahead

Make sure the Inode does not go away while a readahead is in progress.  In
particular:

 - read_async
   - start a readahead
   - get actual read from cache, return
 - close/release
   - call ObjectCacher::release_set() and get unclean > 0, assert

Fixes: #7867
Backport: emperor, dumpling
Signed-off-by: Sage Weil <sage@inktank.com>