commit bcf090e0 upstream.

this was very wrong - mempool_alloc() only guarantees success with GFP_WAIT.
bcache uses GFP_NOWAIT in various other places where we have a fallback,
circuits must've gotten crossed when writing this code or something.
Signed-off-by: Kent Overstreet <kmo@daterainc.com>
Cc: Gabriel de Perthuis <g2p.code@gmail.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>