client: use put_inode on MetaRequest inode refs

When we drop the request inode refs, we need to use put_inode() to ensure
they get cleaned up properly (removed from inode_map, caps released, etc.).
Do this explicitly here (as we do with all other inode put() paths that
matter).

Fixes: #5381
Signed-off-by: Sage Weil <sage@inktank.com>