btrfs: btrfs_root_readonly() broken on big-endian

commit 6ed3cf2c 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>
Cc: Chris Mason <chris.mason@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>