fs: buffer: do not use unnecessary atomic operations when discarding buffers

fs: buffer: do not use unnecessary atomic operations when discarding buffers

Discarding buffers uses a bunch of atomic operations when discarding
buffers because ......  I can't think of a reason.  Use a cmpxchg loop to
clear all the necessary flags.  In most (all?) cases this will be a single
atomic operations.

[akpm@linux-foundation.org: move BUFFER_FLAGS_DISCARD into the .c file]
Signed-off-by: Mel Gorman <redacted>
Cc: Johannes Weiner <redacted>
Cc: Vlastimil Babka <redacted>
Cc: Jan Kara <redacted>
Cc: Michal Hocko <redacted>
Cc: Hugh Dickins <redacted>
Cc: Dave Hansen <redacted>
Cc: Theodore Ts'o <redacted>
Cc: "Paul E. McKenney" <redacted>
Cc: Oleg Nesterov <redacted>
Cc: Rik van Riel <redacted>
Cc: Peter Zijlstra <redacted>
Signed-off-by: Andrew Morton <redacted>
Signed-off-by: Linus Torvalds <redacted>