MMAP Optimization

Enable zfs_getpage, zfs_fillpage, zfs_putpage, zfs_putapage functions.
The functions have been modified to make them Linux friendly.

ZFS uses these functions to read/write the mmapped pages. Using them
from readpage/writepage results in clear code. The patch also adds
readpages and writepages interface functions to read/write list of
pages in one function call.

The code change handles the first mmap optimization mentioned on
https://github.com/behlendorf/zfs/issues/225

Signed-off-by: Prasad Joshi <pjoshi@stec-inc.com>
Signed-off-by: Brian Behlendorf <behlendorf@llnl.gov>
Issue #255