Use swp_swdevt as struct swdevt and swp_swblk as struct swblk function prefix, respectively.

[ota_j.email.ne.jp]

Summary

While working on D23222, I thought it would be a nice idea to gather
similar function prototypes and use the same prefix to indicate
their usages and targets.

Converted swdevt and swblk operating functions with this new prefix and made
their first argument their struct types.

Test Plan

make buildkernel

[ota_j.email.ne.jp]

Updated function name in comment.