zfs: Fix return value of fs_devread()

As evidenced by how other filesystems handle it, a return value of 0
from fs_devread() means failure; nonzero means success. The opposite
assumption was being made in zfs.c for the use of zfs_devread() so fix
the confusion by making zfs_devread() return 0 on success.

It probably doesn't make sense to change the handling of zfs_devread()
in zfs.c instead, because as it is it matches the semantics of the other
functions there.

Signed-off-by: Phaedrus Leeds <mwleeds@mailtundra.com>

fs/zfs/dev.c

@@ -26,5 +26,5 @@ void zfs_set_blk_dev(struct blk_desc *rbdd, struct disk_partition *info)
 int zfs_devread(int sector, int byte_offset, int byte_len, char *buf)
 {
 	return fs_devread(zfs_blk_desc, part_info, sector, byte_offset,
-			  byte_len, buf);
+			  byte_len, buf) ? 0 : 1;
 }