generic/075,112: detect preallocation support for fsx tests

generic/075,112: detect preallocation support for fsx tests

Currently generic/075 and generic/112 have two extra fsx passes each
that exercise fsx with preallocation, which are only enabled for
XFS.

These tests can also be run with other file systems, given that the
XFS prealloc ioctls are implemented in generic code since the
addition of the fallocate system call.  This also means a version of
XFS that does not support preallocation (e.g. because it always
writes out of place) can skip the prealloc tests while still
completing the normal fsx tests just fine.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>