LU-14055 lmv: reduce struct lmv_obd size

LU-14055 lmv: reduce struct lmv_obd size

The lmv_obd struct contains lmv_mdt_descs which is large enough
to reference 512 * 512 = 262144 targets, but there can be only
65536 OSTs or MDTs in a single filesystem today.

Shrink the allocation size to match the current limits, reducing
the size of obd_device.u since this is the largest union member.

This reduces the size of each obd_device from 6752 to 4568 bytes.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I752b021bdb5d02e3ead3bb266121be5dbf3ebbe5
Reviewed-on: https://review.whamcloud.com/41162
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>