KVM: Clean up coalesced MMIO ring full check

Fold coalesced_mmio_has_room() into its sole caller, coalesced_mmio_write(),
as it's really just a single line of code, has a goofy return value, and
is unnecessarily brittle.

E.g. if coalesced_mmio_has_room() were to check ring->last directly, or
the caller failed to use READ_ONCE(), KVM would be susceptible to TOCTOU
attacks from userspace.

Opportunistically add a comment explaining why on earth KVM leaves one
entry free, which may not be obvious to readers that aren't familiar with
ring buffers.

No functional change intended.

Reviewed-by: Ilias Stamatis <[email protected]>
Cc: Paul Durrant <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>

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