Correct make mancheck recipe

The current make recipe for mancheck silently ignores errors. Correct
the recipe so errors cause the mancheck recipe fail.

The zpool reopen command in the zpool.8 manpage had a bullet list
without an .El.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #6790

Makefile.am

@@ -62,9 +62,10 @@ shellcheck:
 
 mancheck:
 	@if type mandoc > /dev/null 2>&1; then \
-		for file in zfs zpool zdb zgenhostid; do \
-			mandoc -Tlint -Werror ${top_srcdir}/man/man8/$$file.8; \
-		done \
+		find ${top_srcdir}/man/man8 -type f -name 'zfs.8' \
+			-o -name 'zpool.8' -o -name 'zdb.8' \
+			-o -name 'zgenhostid.8' | \
+			xargs mandoc -Tlint -Werror; \
 	fi
 
 lint: cppcheck paxcheck