smb3: improve parallel reads of large files

smb3: improve parallel reads of large files

rasize (ra_pages) should be set higher than read size by default
to allow parallel reads when reading large files in order to
improve performance (otherwise there is much dead time on the
network when doing readahead of large files).  Default rasize
to twice readsize.

Signed-off-by: Steve French <stfrench@microsoft.com>