bus: reduce calls to KDBUS_CMD_MEMFD_SIZE_SET ioctl

Instead of calling it for each buffer append, increase allocation
exponentially and set the real value only at the end, when sealing off
the memfd.

This should drastically reduce the number of times we invoke the
ioctl().
Revert "ptyfwd: make master terminal attributes raw, too"

This reverts commit e7d43b3cc30764138c90eaaf95d3d8f49e448890.

This broke the console terminal when booting up a container, so let's
not do this.