ocfs2: Do not map blocks from local quota file on each write

There is no need to map offset of local dquot structure to on disk block
in each quota write. It is enough to map it just once and store the physical
block number in quota structure in memory. Moreover this simplifies locking
as we do not have to take ip_alloc_sem from quota write path.

Acked-by: Joel Becker <Joel.Becker@oracle.com>
Signed-off-by: Jan Kara <jack@suse.cz>