Some minor improvements to vnode_pager_generic_putpages().

• Add asserts that the pages to write are dirty. The last page, if partially written, is only required to be dirty, while completely written pages should have all dirty bit set.
• Use uintmax_t to print vm_page pindexes.
• Use NULL instead of casted zero.
• Remove if () test which duplicated the loop ending condition.
• Miscellaneous style fixes.

Reviewed by: alc, markj (previous version)
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week