In swap_pager_putpages, move the initialization of a free-blocks
counter, and the final freeing of freed swap blocks, outside the
region where an object lock is held. Correct some style(9) and
spelling errors. Change a panic() to a KASSERT(). Change a boolean_t
to a bool.

Suggested by: alc
Reviewed by: alc
Approved by: kib, markj (mentors)
Differential Revision: https://reviews.freebsd.org/D21093