LU-14047 lustre: change EWOULDBLOCK to EAGAIN

LU-14047 lustre: change EWOULDBLOCK to EAGAIN

On linux, EWOULDBLOCK has always been defined as an alias for
EAGAIN. In the interest of readability we should not use two names for
the same thing. So change the remaining uses of EWOULDBLOCK to EAGAIN
and add EWOULDBLOCK||EAGAIN to spelling.txt.

Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: Ib48b8a1e58bfa961d2a4ba411c038c476bfc300d
Reviewed-on: https://review.whamcloud.com/40307
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>