selftests: tmpfs: Add kselftest support to tmpfs

Replace direct error handling with 'ksft_test_result_*'
macros for better reporting.

Test logs:

Before change:
- Without root
 error: unshare, errno 1

- With root
 No, output

After change:
- Without root
TAP version 13
1..1
ok 2 # SKIP This test needs root to run!
Totals: pass:0 fail:0 xfail:0 xpass:0 skip:1 error:0

- With root
TAP version 13
1..1
ok 1 Test : Success
Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0

Link: https://lore.kernel.org/r/20250105085255.124929-3-cvam0000@gmail.com


Signed-off-by: Shivam Chaudhary <cvam0000@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>