selftest/mm: skip if fallocate() is unsupported in gup_longterm

selftest/mm: skip if fallocate() is unsupported in gup_longterm

Currently gup_longterm assumes that filesystems support fallocate() and
uses that to allocate space in files, however this is an optional feature
and is in particular not implemented by NFSv3 which is commonly used in CI
systems leading to spurious failures.  Check for lack of support and
report a skip instead for that case.

Link: https://lkml.kernel.org/r/20250613-selftest-mm-gup-longterm-fallocate-nfs-v1-1-758a104c175f@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>