LU-911 osd: osd_ldiskfs_read returns short reads

LU-911 osd: osd_ldiskfs_read returns short reads

osd_ldiskfs_read returned always the requested size even if short
read occurs hiding possible problems. Now it returns real size.

dt_read() helper returns just the same result and can be used to read
any data.

dt_record_read() must be used only for fixed length records

Fix mdt_getattr_internal() code related to readlink, it always tried
to read link of size + 1, so now we need to take this into account.

Signed-off-by: Mikhail Pershin <tappro@whamcloud.com>
Change-Id: Ib603c5cbe5b06f3f6a9aef74c52bb78cdbd4c3eb
Reviewed-on: http://review.whamcloud.com/2263
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>