uclient: do not accept max_size changes unless they're from auth mds.

uclient: do not accept max_size changes unless they're from auth mds.

Unlike most of the cap options, max_size is an inode member. This meant
that if we got a shared cap grant from a replica MDS, we would set
the max_size to 0!
This caused hangs because when the client would request a new, larger
max_size from the auth MDS then the auth MDS would see the new size
as being smaller than the current max, and drop the message as
being spurious.

Signed-off-by: Greg Farnum <gregory.farnum@dreamhost.com>