nfsd: remove private bin2hex implementation

Calling sprintf in a loop is not very efficient, and in any case,
we already have an implementation of bin-to-hex conversion in lib/
which we might as well use.

Note that original code used to nul-terminate the destination while
bin2hex doesn't. That's why replace kmalloc() with kzalloc().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>