webdav: Do not call into DAVResourceAccess with mutex locked.

This commit cherry-picks 0c3500115c4fd86284a027fc32be704afcf77061
for serf webdav version.

(cherry picked from commit e07cefb4f7ba39d59d25815e208ed61269079142)

Change-Id: I108b0068cad847bf4947ece5e690f789ef034ae9
Reviewed-on: https://gerrit.libreoffice.org/9889
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>