btrfs: btrfs_root_readonly() broken on big-endian

commit 6ed3cf2cdfce4c9f1d73171bd3f27d9cb77b734e upstream.

->root_flags is __le64 and all accesses to it go through the helpers
that do proper conversions.  Except for btrfs_root_readonly(), which
checks bit 0 as in host-endian...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>