nfsd: Fix handling of negative lengths in read_buf()

The length "nbytes" passed into read_buf should never be negative, but
we check only for too-large values of "nbytes", not for too-small
values.  Make nbytes unsigned, so it's clear that the former tests are
sufficient.  (Despite this read_buf() currently correctly returns an xdr
error in the case of a negative length, thanks to an unsigned
comparison with size_of() and bounds-checking in kmalloc().  This seems
very fragile, though.)

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>