Remove all those get_bh()'s and put_bh()'s by extending lock_page() to cover
the troublesome regions.

(get_bh() and put_bh() happen every time whereas contention on a page's lock
in there happens basically never).

Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>