LU-2129 llog: protect llog write against concurrent read

LU-2129 llog: protect llog write against concurrent read

llog_write_rec contains case when buffer passed separately from
header and tail, resulting 3 writes instead of single one, so
concurrent read may read partial record.
Another similar case is llog_pad, which writes header and tail
with required gap to align records at the end of 8K buffer.

Use dt_read/write locks on llog object to protect writes against
read. Such '3-writes' case happens only with configs llogs.

Signed-off-by: Mikhail Pershin <tappro@whamcloud.com>
Change-Id: Idfb94706b2cf8ad22776c1a277b862863f94c2f6
Reviewed-on: http://review.whamcloud.com/4303
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>