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/[e-mail]
Fixes: bd67d5c15cc1 ("Test compaction of mlocked memory")
Signed-off-by: Dev Jain <[e-mail]>
Cc: <[e-mail]>
Cc: Anshuman Khandual <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Cc: Sri Jayaramappa <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

tools/testing/selftests/mm/compaction_test.c

···
 		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",