fs_mgr_fstab: translate reserve_root to reserved_size in fsmgr

Without this patch, fstab should decribe reserve_root=[# of 4KB blocks] in
f2fs mount option as well as reserved_size=[KB] in fsmgr flag to notify it
to framework.
With this patch, it should be just fine to use reserve_root=[# of 4KB blocks]
in fstab.

Bug: 135003600
Test: checked vold.has_reserved
Change-Id: Iab75f7da9792ad205a6c4d2d0f1eba8c16a5266a
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>