sparc: clarify __GFP_NOFAIL allocation

Commit 920c3ed7 ("[SPARC64]: Add basic infrastructure for MD
add/remove notification") has added __GFP_NOFAIL for the allocation
request but it hasn't mentioned why is this strict requirement really
needed.  The code was handling an allocation failure and propagated it
properly up the callchain so it is not clear why it is needed.

Dave has clarified the intention when I tried to remove the flag as not
being necessary:

: It is a serious failure.
:
: If we miss an MDESC update due to this allocation failure, the update
: is not an event which gets retransmitted so we will lose the updated
: machine description forever.
:
: We really need this allocation to succeed.

So add a comment to clarify the nofail flag and get rid of the failure
check because __GFP_NOFAIL allocation doesn't fail.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Cc: David Rientjes <rientjes@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Vipul Pandya <vipul@chelsio.com>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>