imgact_binmisc: abstract away the list lock (NFC)

This module handles relatively few execs (initial qemu-user-static, then
qemu-user-static handles exec'ing itself for binaries it's already running),
but all execs pay the price of at least taking the relatively expensive
sx/slock to check for a match when this module is loaded. Future work will
almost certainly swap this out for another lock, perhaps an rmslock.

The RLOCK/WLOCK phrasing was chosen based on what the callers are really
wanting, rather than using the verbiage typically appropriate for an sx.

MFC after: 1 week