Changes

server: wr_file: Request up to 8 iofd write buffers if available

Writing to disk is usually slower than handling network traffic, so it's
a desirable to increase write buffers to try to submit our write queue
to the kernel/VFS as fast as possible, to avoid it growing too much.
This should also improve CPU use at the expense of some dozen KBs of
extra use, which in the server should be plenty available.

Using up to 8 buffer sounds like a good start, being it a good
compromise between memory requirements and batching of msgbs. It also
turns outs it's the maximum amount of buffers allowed by osmo_io current
implementation ;)

Depends: libosmocore.git 378cf564c8859311415aa70adeb220cedbe56eb3
Change-Id: I167e5f9b7f9d5693b3df05f713288c741031c532