nilfs2: simplify nilfs_get_page function

Implementation of nilfs_get_page() is a bit old as below:

 - A common read_mapping_page inline function is now available instead
   of its read_cache_page use.
 - wait_on_page_locked() use in the function is eliminable since
   read_cache_page function does the same thing through wait_on_page_read().
 - PageUptodate() check is eliminable for the same reason.

This renews nilfs_get_page() based on these points.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>