Allow disabling of CoW to fail when not supported by filesystem

Summary:
It is fine to neither support reading of attrs nor setting the NO_COW
flag. The latter only happens when the filesystem does not use CoW at
all.

Test Plan:
make
TMPDIR=<dir on XFS> ctest
TMPDIR=<dir on BTRFS> ctest
strace -eioctl -f ctest -R transactiontest

Works on BTRFS and TMPFS, silently ignores EOPNOTSUPP on XFS (without reflink support)

Reviewers: Baloo, adridg, Frameworks, michaelh

Reviewed By: Baloo, adridg, michaelh

Subscribers: bcooksley, adridg, FreeBSD, Frameworks

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D12135