OSHMEM: sshmem verbs: workaround shared_mr procfs bug

dereg shared_mr before doing dereg on its mr.
(cherry picked from commit cd67642183954d5dad560d907ded422d47ee50d4)
open-mpi/ompi@cd67642183954d5dad560d907ded422d47ee50d4
@miked-mellanox @jladd-mlnx