nfsd: Add support for the SP4_MACH_CRED case in ExchangeID

Commit f4179ad46fa4 added support for operation bitmaps for
NFSv4.1/4.2. This commit uses those to implement the SP4_MACH_CRED
case for the NFSv4.1/4.2 ExchangeID operation since the Linux
NFSv4.1/4.2 client is now using this for Kerberized mounts.
The Linux Kerberized NFSv4.1/4.2 mounts currently work without
support for this because Linux will fall back to SP4_NONE,
but there is no guarantee this fallback will work forever.

This commit only affects Kerberized NFSv4.1/4.2 mounts from
Linux at this time.

MFC after: 3 months