Make smgr access for a BufferManagerRelation safer in relcache inval

Make smgr access for a BufferManagerRelation safer in relcache inval

Currently there's no bug, because we have no code path where we
invalidate relcache entries where it'd cause a problem.  But it's more
robust to do it this way in case we introduce such a path later, as some
Postgres forks reportedly already have.

Author: Daniil Davydov <3danissimo@gmail.com>
Reviewed-by: Stepan Neretin <slpmcf@gmail.com>
Discussion: https://postgr.es/m/CAJDiXgj3FNzAhV+jjPqxMs3jz=OgPohsoXFj_fh-L+nS+13CKQ@mail.gmail.com