In these instances, we are trying to determine if a page has been accessed
since we began the operation for the sake of retry.  This is easily
accomplished by doing a gang lookup in the page mapping radix tree, and it
saves us the dependency on the flag (so that we might eventually delete
it).

btrfs_page_exists_in_range borrows heavily from find_get_page, replacing
the radix tree look up with a gang lookup of 1, so that we can find the
next highest page >= index and see if it falls into our lock range.

Signed-off-by: Chris Mason <clm@fb.com>
Signed-off-by: Alex Gartrell <agartrell@fb.com>