util: Avoid signed/unsigned integer comparisons

util: Avoid signed/unsigned integer comparisons

../../../lib/util/sys_rw.c: In function ‘sys_pread_full’:
../../../lib/util/sys_rw.c:219:25: warning: comparison of integer expressions of different signedness: ‘ssize_t’ {aka ‘long int’} and ‘size_t’ {aka ‘long unsigned int’} [-Wsign-compare]
  219 |                 if (ret > curr_count) {
      |                         ^
../../../lib/util/sys_rw.c: In function ‘sys_pwrite_full’:
../../../lib/util/sys_rw.c:282:25: warning: comparison of integer expressions of different signedness: ‘ssize_t’ {aka ‘long int’} and ‘size_t’ {aka ‘long unsigned int’} [-Wsign-compare]
  282 |                 if (ret > curr_count) {
      |                         ^
../../../lib/util/sys_rw.c: In function ‘sys_write_full’:
../../../lib/util/sys_rw.c:321:25: warning: comparison of integer expressions of different signedness: ‘ssize_t’ {aka ‘long int’} and ‘size_t’ {aka ‘long unsigned int’} [-Wsign-compare]
  321 |                 if (ret > curr_count) {
      |                         ^

Signed-off-by: Martin Schwenke <mschwenke@ddn.com>
Reviewed-by: Volker Lendecke <vl@samba.org>