segment: fix read/write g->dlen race, avoid copying kernel memory, qlock

code like "return g->dlen;" is wrong as we do not hold the
qlock of the global segment. another process could come in
and override g->dlen making us return the wrong byte count.

avoid copying when we already got a kernel address (kernel memory
is the same on processes) which is the case with bread()/bwrite().
this is the same optimization that devsd does.

also avoid allocating/freeing and copying while holding the qlock.
when we copy to/from user memory, we might fault preventing
others from accessing the segment while fault handling is in
progress.

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