xfs/054: only filter ROOT_INO at the beginning

xfs/054: only filter ROOT_INO at the beginning

In the test ROOT_INO is filtered out and/or replaced, but if
ROOT_INO is also 32, more "32"s are filtered and replaced than
expected. This happens to me when testing 512B block size XFS and 1k
block size CRC enabled XFS.

Fix it by filtering out only ROOT_INO at the beginning of a line,
and removing all "g" modifiers in sed expressions.

Also the ROOT_INO should be the root inode number of TEST_DIR not
SCRATCH_MNT.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>