Fix sign-compare warnings in pg_iovec.h.

The code in question (pg_preadv() and pg_pwritev()) has been around
for a while, but commit 15c9ac3629 moved it to a header file.  If
third-party code that includes this header file is built with
-Wsign-compare on a system without preadv() or pwritev(), warnings
ensue.  This commit fixes said warnings by casting the result of
pg_pread()/pg_pwrite() to size_t, which should be safe because we
will have already checked for a negative value.

Author: Wolfgang Walther
Discussion: https://postgr.es/m/16989737-1aa8-48fd-8dfe-b7ada06509ab%40technowledgy.de
Backpatch-through: 17

src/include/port/pg_iovec.h

@@ -68,7 +68,7 @@ pg_preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset)
 		}
 		sum += part;
 		offset += part;
-		if (part < iov[i].iov_len)
+		if ((size_t) part < iov[i].iov_len)
 			return sum;
 	}
 	return sum;
@@ -107,7 +107,7 @@ pg_pwritev(int fd, const struct iovec *iov, int iovcnt, off_t offset)
 		}
 		sum += part;
 		offset += part;
-		if (part < iov[i].iov_len)
+		if ((size_t) part < iov[i].iov_len)
 			return sum;
 	}
 	return sum;