e2scrub_all: fix "e2scurb_all -r"

e2scrub_all: fix "e2scurb_all -r"

The e2scrub_all program was broken by commit c7d6525ecaab
("e2scrub_all: refactor device probe loop") so that it would use the
path of the snapshot volume instead of the base volume.  This caused
"e2scrub_all -r" to pass the wrong pathname to e2scrub, with the
result that e2scrub would abort with an error instead of removing the
snapshot volume.

Fixes: c7d6525ecaab ("e2scrub_all: refactor device probe loop")
Addresses-Debian-Bug: #931679
Signed-off-by: Theodore Ts'o <tytso@mit.edu>