xfstests: do not unmount tmpfs during remount

xfstests: do not unmount tmpfs during remount

Several tests unmount then re-mount the scratch filesystem, to check
that the content is unchanged; but unmounting a tmpfs is designed to
lose its content, which causes such tests to fail unnecessarily.

Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Junho Ryu <jayr@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>