lvresize of CoW snapshot incorrectly applies filesystem resize checks

[bmr-cymru]

Resizing a CoW snapshot LV with lvresize allows the snapshot exception store to be extended. Attempting to shrink a CoW snapshot with a file system present incorrectly applies the filesystem resize checks as though the device size was being modified rather than the exception store:

# lvcreate -n test1 -L 1G fedora
  Logical volume "test1" created.

# mkfs.xfs /dev/fedora/test1
Or:
# mkfs.ext4 /dev/fedora/test1

# lvcreate -s -n test1-snap -L 1G fedora/test1
  Logical volume "test1-snap" created.

Attempting to shrink the CoW exception store fails with:

XFS:
# lvresize -L512M fedora/test1-snap
  File system xfs found on fedora/test1-snap.
  File system size (1.00 GiB) is larger than the requested size (512.00 MiB).
  File system reduce is required and not supported (xfs).

Ext4:
# lvresize -L512M fedora/test1-snap
  File system ext4 found on fedora/test1-snap.
  File system size (1.00 GiB) is larger than the requested size (512.00 MiB).
  File system reduce is required (see resize2fs or --resizefs.)

The check that should be applied would determine whether it's safe to truncate the CoW store (i.e. fail if any exceptions are allocated beyond the new size limit).