LU-11956 mdd: do not reset original lu_buf.lb_len

In mdd_iterate_xattrs(), we are resetting the xbuf.lb_len
to a smaller value returned by linkea_overflow_shrink().

If that's the last xattr we gonna process, we could deduct
less than originally allocated size from obd_memory stats,
failing the memleak check later.

Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Change-Id: I6175a91c61ceb0e37ab889d0cfd904f4993ab5cc
Reviewed-on: https://review.whamcloud.com/35333
Reviewed-by: Li Xi <lixi@ddn.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>