ceph: clean up handle_cap_grant, handle_caps wrt session mutex

Drop session mutex unconditionally in handle_cap_grant, and do the
check_caps from the handle_cap_grant helper.  This avoids using a magic
return value.

Also avoid using a flag variable in the IMPORT case and call
check_caps at the appropriate point.

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