IB/srp: Avoid problems if a header uses pr_fmt

SRP defines pr_fmt(fmt) to be "PFX fmt", and then includes a bunch of
header files before it gets around to defining PFX.  This causes
problems if any of the header files do a pr_... and use pr_fmt().

Fix this by using KBUILD_MODNAME instead of the private PFX.

Acked-by: Chris Metcalf <cmetcalf@tilera.com>

Signed-off-by: Roland Dreier <roland@purestorage.com>