generic/300: add fallocate() checks

generic/300: add fallocate() checks

Generic/300 fails when run on a test filesystem that does not
support fallocate(). It uses fio's falloc ioengine to generate part
of its I/O load and both allocates blocks and punches holes.  This
causes EOPNOTSUPP failures when the test is run on indirect
block-mapped ext4 filesystems or pre-3.14 ext4 filesystems created
with bigalloc.

Verify that the test filesystem supports fallocate() before
proceeding with the test, checking for both block allocation and
hole punching capabilities.  Also, delete any pre-existing test
output.

Signed-off-by: Eric Whitney <enwlinux@gmail.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>