net/mlx4: refactor interrupt FD settings

net/mlx4: refactor interrupt FD settings

File descriptors used for interrupts processing must be made non-blocking.

Doing so as soon as they are opened instead of waiting until they are
needed is more efficient as it avoids performing redundant system calls and
run through their associated error-handling code later on.

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>