selftests: hugetlb_dio: check for initial conditions to skip in the start

The test should be skipped if initial conditions aren't fulfilled in the
start instead of failing and outputting non-compliant TAP logs.  This kind
of failure pollutes the results.  The initial conditions are:

- The test should only execute if /tmp file can be allocated.
- The test should only execute if huge pages are free.

Before:
TAP version 13
1..4
Bail out! Error opening file
: Read-only file system (30)
 # Planned tests != run tests (4 != 0)
 # Totals: pass:0 fail:0 xfail:0 xpass:0 skip:0 error:0

After:
TAP version 13
1..0 # SKIP Unable to allocate file: Read-only file system

Link: https://lkml.kernel.org/r/20241101141557.3159432-1-usama.anjum@collabora.com
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Fixes: 3a103b5 ("selftest: mm: Test if hugepage does not get leaked during __bio_release_pages()")
Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Donet Tom <donettom@linux.ibm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: musamaanjum

tools/testing/selftests/mm/hugetlb_dio.c

@@ -44,13 +44,6 @@ void run_dio_using_hugetlb(unsigned int start_off, unsigned int end_off)
 	if (fd < 0)
 		ksft_exit_fail_perror("Error opening file\n");
 
-	/* Get the free huge pages before allocation */
-	free_hpage_b = get_free_hugepages();
-	if (free_hpage_b == 0) {
-		close(fd);
-		ksft_exit_skip("No free hugepage, exiting!\n");
-	}
-
 	/* Allocate a hugetlb page */
 	orig_buffer = mmap(NULL, h_pagesize, mmap_prot, mmap_flags, -1, 0);
 	if (orig_buffer == MAP_FAILED) {
@@ -94,8 +87,20 @@ void run_dio_using_hugetlb(unsigned int start_off, unsigned int end_off)
 int main(void)
 {
 	size_t pagesize = 0;
+	int fd;
 
 	ksft_print_header();
+
+	/* Open the file to DIO */
+	fd = open("/tmp", O_TMPFILE | O_RDWR | O_DIRECT, 0664);
+	if (fd < 0)
+		ksft_exit_skip("Unable to allocate file: %s\n", strerror(errno));
+	close(fd);
+
+	/* Check if huge pages are free */
+	if (!get_free_hugepages())
+		ksft_exit_skip("No free hugepage, exiting\n");
+
 	ksft_set_plan(4);
 
 	/* Get base page size */