mbuf(9): Fix size of mbuf for all 32-bit platforms (i386, ARM, PowerPC and RISCV)

Do this by reducing the size of the MBUF_PEXT_MAX_PGS, causing "struct mbuf" to
be bigger than M_SIZE, and also add a missing padding field to ensure 64-bit
alignment.

Reviewed by: gallatin@
Reported by: Elliott Mitchell
Differential revision: https://reviews.freebsd.org/D35339
MFC after: 1 week
Sponsored by: NVIDIA Networking