mm: simplify filemap_nopage

Identical block is duplicated twice: contrary to the comment, we have been
re-reading the page *twice* in filemap_nopage rather than once.

If any retry logic or anything is needed, it belongs in lower levels anyway.
Only retry once.  Linus agrees.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>