btrfs: factor out allocating an array of pages

Several functions currently populate an array of page pointers one
allocated page at a time. Factor out the common code so as to allow
improvements to all of the sites at once.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>