dp: set be_ctx->provider as part of dp_init request
Backend context is overused inside sssd code even during its initialization.
Some parts of initialization code requires access to be_ctx->provider so we
must make it available as soon as possible.
Better solution would be to always use 'provider' directly in initialization
but this makes it safer for any future changes as one does not have to keep
in mind when it is safe to use be_ctx->provider and when not. Now it is
always safe.
Resolves:
https://pagure.io/SSSD/sssd/issue/3812
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>