block: cleanup bio_alloc_bioset()

this warning (which got fixed by commit b2bf9683):

  fs/bio.c: In function ‘bio_alloc_bioset’:
  fs/bio.c:305: warning: ‘p’ may be used uninitialized in this function

Triggered because the code flow in bio_alloc_bioset() is correct
but a bit complex for the compiler to see through.

Streamline it a bit - this also makes the code a tiny bit more compact:

   text	   data	    bss	    dec	    hex	filename
   7540	    256	     40	   7836	   1e9c	bio.o.before
   7539	    256	     40	   7835	   1e9b	bio.o.after

Also remove an older compiler-warnings annotation from this function,
it's not needed.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>