rpc: warn and truncate grouplist if RPC/AUTH can not hold everything

The GlusterFS protocol currently uses AUTH_GLUSTERFS_V2 in the RPC/AUTH
header. This header contains the uid, gid and auxiliary groups of the
user/process that accesses the Gluster Volume.

The AUTH_GLUSTERFS_V2 structure allows up to 65535 auxiliary groups to
be passed on. Unfortunately, the RPC/AUTH header is limited to 400 bytes
by the RPC specification: http://tools.ietf.org/html/rfc5531#section-8.2

In order to not cause complete failures on the client-side when trying
to encode a AUTH_GLUSTERFS_V2 that would result in more than 400 bytes,
we can calculate the expected size of the other elements:

    1 | pid
    1 | uid
    1 | gid
    1 | groups_len
   XX | groups_val (GF_MAX_AUX_GROUPS=65535)
    1 | lk_owner_len
   YY | lk_owner_val (GF_MAX_LOCK_OWNER_LEN=1024)
  ----+-------------------------------------------
    5 | total xdr-units

  one XDR-unit is defined as BYTES_PER_XDR_UNIT = 4 bytes
  MAX_AUTH_BYTES = 400 is the maximum, this is 100 xdr-units.
  XX + YY can be 95 to fill the 100 xdr-units.

  Note that the on-wire protocol has tighter requirements than the
  internal structures. It is possible for xlators to use more groups and
  a bigger lk_owner than that can be sent by a GlusterFS-client.

This change prevents overflows when allocating the RPC/AUTH header. Two
new macros are introduced to calculate the number of groups that fit in
the RPC/AUTH header, when taking the size of the lk_owner in account. In
case the list of groups exceeds the maximum possible, only the first
groups are passed over the RPC/GlusterFS protocol to the bricks.
A warning is added to the logs, so that most system administrators will
get informed.

The reducing of the number of groups is not a new inventions. The
RPC/AUTH header (AUTH_SYS or AUTH_UNIX) that NFS uses has a limit of 16
groups. Most, if not all, NFS-clients will reduce any bigger number of
groups to 16. (nfs.server-aux-gids can be used to workaround the limit
of 16 groups, but the Gluster NFS-server will be limited to a maximum of
93 groups, or fewer in case the lk_owner structure contains more items.)

Change-Id: I8410e59d0fd246d601b54b961d3ae9cb5a858c10
BUG: 1053579
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/7202
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-by: Santosh Pradhan <spradhan@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>