fuse2fs: don't let ENOENT escape from ioctl_fitrim

fuse2fs: don't let ENOENT escape from ioctl_fitrim

If ext2fs_find_first_set_block_bitmap2 returns ENOENT to indicate
that there are no bits set between start and b, then the entire region
is free space and we can discard it.  In that case, zero out err so that
it won't get returned as an error.

Cc: <linux-ext4@vger.kernel.org> # v1.43
Fixes: 81cbf1ef4f5dab ("misc: add fuse2fs, a FUSE server for e2fsprogs")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>