LU-911 osd: zerocopy methods in ldiskfs osd

LU-911 osd: zerocopy methods in ldiskfs osd

add implementation for zerooopy methods to manipulate data:
grab/release buffers, prepare them for read/write, commit
modified buffers, truncate

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: Ied662c7c837bb25b71587b69096bc12fe002d115
Reviewed-on: http://review.whamcloud.com/1834
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Mike Pershin <tappro@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>