Remove two early-development BUG_ONs from page_add_file_rmap.

The pfn_valid test (originally useful for checking that nobody passed an
artificial struct page) comes too late, since we already have the struct
page.

The PageAnon test (useful when anon was first distinguished from file rmap)
prevents ->nopage implementations from reusing ->mapping, which would
otherwise be available.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>