xfstests: added -P option to $DF_PROG

xfstests: added -P option to $DF_PROG

Added -P option to $DF_PROG and changed the invocation of
'df' command in generic/{251,260,273,275} testcases
with $DF_PROG.

Otherwise the testcases will fail if the scratch
device has a long name (for example, if it's an LVM volume).
Because df outputs its usage stats with two lines:

/dev/mapper/xfstests-disk1
                       3030800      4608   2868908   1% /tmp/mnt/disk1

Signed-off-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>