Stop checking TD_IDLETHREAD() in buffer cache routines.

These predicates are vestigal and cannot be true today. For example,
idle threads are not allowed to acquire locks.

Also cache curthread in breada().

No functional change intended.

Reviewed by: kib, mckusick
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20066