e2scrub_all: refactor device probe loop

e2scrub_all: refactor device probe loop

Paul Menzel reported that the e2scrub_all reaper service that runs at
startup takes a long time to run, and Ted Ts'o pointed out that we could
do a lot less work by using lvs as the outer loop in the ext4 filesystem
probe function so that we only have to lsblk the lvm devices containing
ext4 filesystems.

Therefore, refactor the loops to put lvs first, which should boost speed
a bit.

[ Made some of the further optimizations suggested by Lukas Czerner.  -- TYT ]

Reported-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>