ceph: fix inode removal from snap realm when racing with migration

When an inode was dropped while being migrated between two MDSs,
i_cap_exporting_issued was non-zero such that issue caps were non-zero and
__ceph_is_any_caps(ci) was true.  This prevented the inode from being
removed from the snap realm, even as it was dropped from the cache.

Fix this by dropping any residual i_snap_realm ref in destroy_inode.

Signed-off-by: Sage Weil <sage@newdream.net>