useradd: check return value from chmod and log it

useradd: check return value from chmod and log it

covscan was complaining abot calling chmod and ignoring the return
value:
Error: CHECKED_RETURN (CWE-252):
shadow-4.6/src/useradd.c:2084: check_return: Calling
"chmod(prefix_user_home, mode)" without checking return value. This
library function may fail and return an error code.
2082|   		mode_t mode = getdef_num ("HOME_MODE",
2083|   		                          0777 & ~getdef_num ("UMASK", GETDEF_DEFAULT_UMASK));
2084|-> 		chmod (prefix_user_home, mode);
2085|   		home_added = true;
2086|   #ifdef WITH_AUDIT