Eliminate use of M_EXT in IP6_EXTHDR_CHECK() by trimming a redundant
'if'/'else' case: it matches the simple 'else' case that follows.

This reduces awareness of external-storage mechanics outside of the
mbuf allocator.

Reviewed by: bz
MFC after: 3 days
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D900