client: Ensure inode/dentries are ref counted

client: Ensure inode/dentries are ref counted

The MetaRequest holds onto inodes and dentries
for retrying unsafe requests, but those objects
might be removed from the cache (unlink for example)
causing the inode/dentry to be freed.  Ensure that
the inode/dentry is never freed while the MetaRequest
holds onto it by putting/getting the refs using
set/get interfaces.

Signed-off-by: Sam Lang <sam.lang@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>