Currently, MCA entries remain on an every-growing linked list. This means
that it becomes increasingly expensive to process a steady stream of
correctable errors. Additionally, the memory used by the MCA entries can
grow without bound.

Change the code to maintain two separate lists: a list of entries which
still need to be logged, and a list of entries which have already been
logged. Additionally, allow a user-configurable limit on the number of
entries which will be saved after they are logged. (The limit defaults
to -1 [unlimited], which is the current behavior.)

Reviewed by: imp, jhb
MFC after: 2 weeks
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D20482