Reduce the amount of calls to VOP_BMAP() made from the local vnode
pager. It is enough to execute VOP_BMAP() once to obtain both the
disk block address for the requested page, and the before/after limits
for the contiguous run. The clipping of the vm_page_t array passed to
the vnode_pager_generic_getpages() and the disk address for the first
page in the clipped array can be deduced from the call results.

While there, remove some noise (like if (1) {...}) and adjust nearby
code.

Reviewed by: alc
Discussed with: glebius
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 3 weeks