subr_uio.c: Remove a KASSERT() for large NFS server I/O

When the NFS server is set to allow an I/O size greater
than 1Mbyte (not allowed in FreeBSD's main yet), a
KASSERT() in allocuio() can fail when:
zfs_freebsd_write()->zfs_write()->zfs_uiocopy()
->cloneuio()->allocuio()
is called for a large NFS server write.

Since the userland API callers to allocuio() already
check that the size does not exceed UIO_MAXIOV,
there does not seem to be a need to a KASSERT()
here.

Removing the KASSERT() allows NFS server writes
of greater than 1Mbyte to work, once the NFS code
is patched to allow them.

Reviewed by: kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D57005