NFSv4: Fix up decode_attr_mdsthreshold

Fix an incorrect use of 'likely()'. The FATTR4_WORD2_MDSTHRESHOLD
bit is only expected in NFSv4.1 OPEN calls, and so is actually
rather _unlikely_.

decode_attr_mdsthreshold needs to clear FATTR4_WORD2_MDSTHRESHOLD
from the attribute bitmap after it has decoded the data.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: Andy Adamson <andros@netapp.com>