nfsd4: fix seqid_mutating_error

The set of errors here does *not* agree with the set of errors specified
in the rfc!

While we're there, turn this macros into a function, for the usual
reasons, and move it to the one place where it's actually used.

Cc: stable@kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>