ceph: set io_pages bdi hint

This patch sets the io_pages bdi hint based on the rsize mount option.
Without this patch large buffered reads (request size > max readahead)
are processed sequentially in chunks of the readahead size (i.e. read
requests are sent out up to the readahead size, then the
do_generic_file_read() function waits until the first page is received).

With this patch read requests are sent out at once up to the size
specified in the rsize mount option (default: 64 MB).

Signed-off-by: Andreas Gerstmayr <andreas.gerstmayr@catalysts.cc>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Yan, Zheng <zyan@redhat.com>