nfsd: better layoutupdate bounds-checking

You could add any multiple of 2^32/PNFS_SCSI_RANGE_SIZE to nr_iomaps and
still pass this check.  You'd probably still fail the following kcalloc,
but best to be paranoid since this is from-the-wire data.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>