Need special read/write/getsize IRX for PS2. Budget is ~20kb total code+runtime ram

[sahlberg]

Tweak smb2_context. It is ~7kb in size, ~6kb of which is the iovector max size: #define SMB2_MAX_VECTORS 256

For special IRX, trim PS2 IRX build to have a smb2_context of < 512 bytes.

Need a special usmb2_context that only contins the fd and hte se number, minimum we need to do single threaded ead/write/stat calls

See https://github.com/rickgaiser/neutrino/issues/109#issuecomment-2939120618