NAS-139627 / 26.0.0-BETA.1 / NTB: ntb_transport: Preallocate memory windows

[amotin]

Original Linux code allocated memory on every link up and freed on every link down. It may be a problem to allocate several MBs of physically contiguous memory on a running system. To workaround that, speculatively pre-allocate it on boot and then reallocate only if remote host requests different parameters, which should be very rare.

This replicates the logic we used on FreeBSD.

[claude]

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[bugclerk]

Jira URL: https://ixsystems.atlassian.net/browse/NAS-139627

[claude]

LGTM!

Reviewed the NTB transport memory window DMA buffer pre-allocation changes, and everything looks solid. The approach to allocate buffers at probe time and maintain them across link cycles should effectively prevent the expensive contiguous memory allocation failures.

[bugclerk]

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