ANDROID: f2fs: fixup ABI break due to reserved_pin_section

Take advantage of a hole in f2fs_sb_info to add reserved_pin_section
member and avoid an ABI break.

Bug: 428889879
Cc: Paul Lawrence <paullawrence@google.com>
Cc: Giuliano Procida <gprocida@google.com>
Cc: Todd Kjos <tkjos@google.com>
Fixes: 12727f8a ("FROMGIT: f2fs: introduce reserved_pin_section sysfs entry")
Change-Id: Id27596b073751d94bfefa0c2001b8933a4bed15c
Signed-off-by: Carlos Llamas <cmllamas@google.com>