Merge pull request #16188 from bangerth/split-2

Split a mutex into two that each guard one set of variables.

diff --cc source/fe/fe_dgq.cc
Simple merge

diff --cc source/fe/fe_q_base.cc
Simple merge

diff --cc source/fe/fe_raviart_thomas_nodal.cc
Simple merge

diff --cc source/fe/fe_simplex_p.cc
Simple merge