nfs: fix port value parsing

The valid values of nfs options port and mountport are 0 to USHRT_MAX.

The fs parser will return a fail for port values that are negative
and the sloppy option handling then returns success.

But the sloppy option handling is meant to return success for invalid
options not valid options with invalid values.

Restricting the sloppy option override to handle failure returns for
invalid options only is sufficient to resolve this problem.

Changes:

v2: utilize the return value from fs_parse() to resolve this problem
    instead of changing the parameter definitions.

Suggested-by: Trond Myklebust <trondmy@hammerspace.com>
Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>