common/defrag: skip defrag tests on DAX-enabled filesystems

common/defrag: skip defrag tests on DAX-enabled filesystems

Online defragmentation is not supported on ext4 DAX-enabled filesystems.
The ext4 defrag ioctl (EXT4_IOC_MOVE_EXT) returns EOPNOTSUPP when used
on DAX files.

Add an ext4-specific check in _require_defrag() to skip tests when DAX
is enabled, avoiding false failures on ext4/301-304, ext4/308, and
generic/018.

XFS defrag works with DAX, so this check is ext4-specific.

Suggested-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Disha Goel <disgoel@linux.ibm.com>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: Zorro Lang <zlang@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>