selftests/mm: compaction_test: fix incorrect write of zero to nr_hugepages

Currently, the test tries to set nr_hugepages to zero, but that is not
actually done because the file offset is not reset after read().  Fix that
using lseek().

Link: https://lkml.kernel.org/r/20240521074358.675031-3-dev.jain@arm.com
Fixes: bd67d5c ("Test compaction of mlocked memory")
Signed-off-by: Dev Jain <dev.jain@arm.com>
Cc: <stable@vger.kernel.org>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Sri Jayaramappa <sjayaram@akamai.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Dev Jain

tools/testing/selftests/mm/compaction_test.c

@@ -108,6 +108,8 @@ int check_compaction(unsigned long mem_free, unsigned long hugepage_size)
 		goto close_fd;
 	}
 
+	lseek(fd, 0, SEEK_SET);
+
 	/* Start with the initial condition of 0 huge pages*/
 	if (write(fd, "0", sizeof(char)) != sizeof(char)) {
 		ksft_print_msg("Failed to write 0 to /proc/sys/vm/nr_hugepages: %s\n",