Btrfs: check prepare_uptodate_page() error code earlier

Btrfs: check prepare_uptodate_page() error code earlier

prepare_pages() may end up calling prepare_uptodate_page() twice if our
write only spans a single page.  But if the first call returns an error,
our page will be unlocked and its not safe to call it again.

This bug goes all the way back to 2011, and it's not something commonly
hit.

While we're here, add a more explicit check for the page being truncated
away.  The bare lock_page() alone is protected only by good thoughts and
i_mutex, which we're sure to regret eventually.

Reported-by: Dave Jones <dsj@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>
(cherry picked from commit bb1591b4ea1a1485ebc79be4e4748e94f96c670b)
(cherry picked from commit 3684e859ba549093a662b89fcae2bb7b89e87a73)