Inline four often-called new functions

The BDAT state switchers should happen so often during SMTP reception that a
compiler hint to inline seems wise.

The length filter checks happen on every start-up, which for Exim is often
enough that I think an inline these is warranted too.

(cherry picked from commit 6e3d0e3f1c8228ef19a3d1ba61f131cef3172ceb)
(cherry picked from commit 6ca5fcba34070f4495a0188f16eb2e4d78f3430a)