[roll] Roll fuchsia [fxfs] Move bitmap-based zeroing out of read_and_decrypt.

Externalized to be more in-keeping with "one function, one purpose".

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1276525
Original-Revision: e6cd0bc73c35c16c23e7e7c38a415feac85adb52
GitOrigin-RevId: 48b9eb5d264847a63f90926a45771edddf555088
Change-Id: I58afd61871998acdead4b3165e29169b83c27e7e

Author: Aaron Drew

stem

@@ -8,7 +8,7 @@
              root=""
              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="df47ba730d90b86535804762697d89d8bca819e0"
+             revision="e6cd0bc73c35c16c23e7e7c38a415feac85adb52"
              />
   </imports>
   <projects>