mpool/mpool_get.c: Avoid clobbering 'errno' when handling 'pread' errors

POSIX.1-2024 states that the 'free' function "shall not modify errno if
ptr is a null pointer or a pointer previously returned as if by malloc()
and not yet deallocated". However this is a fairly recent addition
and non-compliant allocators might still clobber 'errno', causing
'mpool_get' to return the wrong error code. Fix this by saving
and restoring 'errno' after calling 'free'.

Sponsored by:	Klara, Inc.
Reviewed by:	obiwac
Differential Revision:	https://reviews.freebsd.org/D55463
MFC after:	1 week

lib/libc/db/mpool/mpool.c

@@ -214,10 +214,14 @@ mpool_get(MPOOL *mp, pgno_t pgno,
 	/* Read in the contents. */
 	off = mp->pagesize * pgno;
 	if ((nr = pread(mp->fd, bp->page, mp->pagesize, off)) != (ssize_t)mp->pagesize) {
+		int serrno;
+
 		switch (nr) {
 		case -1:
 			/* errno is set for us by pread(). */
+			serrno = errno;
 			free(bp);
+			errno = serrno;
 			mp->curcache--;
 			return (NULL);
 		case 0: