fuse2fs: fix incorrect unit conversion at the end of FITRIM

fuse2fs: fix incorrect unit conversion at the end of FITRIM

generic/260 also points out that the bytes cleared are in the wrong
units -- they're supposed to be in bytes, but the "cleared" variable is
in units of fsblocks.  Fix that.

Cc: linux-ext4@vger.kernel.org # v1.47.3-rc1
Fixes: 7235b58533b9cd ("fuse2fs: fix FITRIM validation")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/175182663095.1984706.13547873393931840622.stgit@frogsfrogsfrogs
Signed-off-by: Theodore Ts'o <tytso@mit.edu>