LU-11868 osd: Set max ea size to XATTR_SIZE_MAX

Lustre currently limits EA size to either ~1 MiB (ldiskfs)
or 32K (ZFS).  VFS has its own limit, XATTR_SIZE_MAX,
which we must respect to interoperate correctly with
userspace tools like tar, getattr, and the getxattr()
syscall.

Set this as the new max EA size for both ldiskfs and ZFS.

(The current 32K on ZFS is too small for
LOV_MAX_STRIPE_COUNT [2000] files, so needs to be raised
regardless.)

In order to use this correctly, we have to use the real ea
size on the client.  The previous code for maximum ea size
on the client (KEY_MAX_EASIZE, llite.max_easize) used a
calculated value based on number of targets.

With one exception, the mdc code already uses the default
ea size rather than the max.  Default ea size adjusts
automatically to the largest size sent by the server.

The exception is the open code, which uses the max so it
never has to resend a layout request.  This patch changes
it to use default, which means that the first time a very
widely striped file is opened, the open will be resent.

Add limit checks on client & server so the xattr size limit
is honored.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I4da62691f30fa276d20959810116cf558cccc515
Reviewed-on: https://review.whamcloud.com/34058
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alexandr Boyko <c17825@cray.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>