client: add a new inode release request callback

client: add a new inode release request callback

trim_caps() walks the list of caps on the session, and releases
non-auth caps, and attempts to trim dentries until the cache
size is under the max_caps value requested by MDS.

This is fine for FUSE, but doesn't really match the use-case of
nfs-ganesha. Ganesha typically looks up inodes by inode number, not
by dentry. It's quite possible that after a restart, we may have a
ton of outstanding inodes with no dentries associated with them.

Ganesha holds a reference to each inode, so libcephfs can't release
them, and we don't have a way to request that ganesha do so.

Add a new ino_release_callback and finisher. The intent is to allow
libcephfs to "upcall" to the application and request that it release
references to a specific inode.

Signed-off-by: Jeff Layton <jlayton@redhat.com>