xfstests 276: fix error 'FIBMAP: Invalid argument'

Commit 05dadc0 ("Btrfs: add fiemap's flag check") added validity
checks to the fiemap flags and hence invalid flags are now being
rejected. Test 276 passes an invalid fiemap flag to btrfs, and so
that test fails since this commit was added.

Btrfs doesn't support FIEMAP_FLAG_XATTR, which is enabled by -x option
of filefrag, and will fail with
	'FIBMAP: Invalid argument'
for 'filefrag -vx'. 'filefrag -vx' fails on btrfs with
     'FIEMAP failed with unsupported flags 2'
Remove the '-x' option.

Signed-off-by: Wang Sheng-Hui <shhuiw@gmail.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>