Rename m_unmappedtouio() to m_unmapped_uiomove().

This function doesn't only copy data into a uio but instead is a
variant of uiomove() similar to uiomove_fromphys().

Reviewed by: gallatin, markj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30444

(cherry picked from commit aa341db39b6373c5e242f376a3cabe6a6b99141e)