support f2fs device aliasing feature

We can hold the storage space of one of f2fs multi-device for data
partition, while allowing the space to be used by another purpose.
f2fs will create a sequential and pinned file as a placeholder and it
can be deallocated and returned back to data partition after it is done
with using it. We can create the file, as we define the device with
"exp_alias:" tag in the fstab.

Bug: 336319772
Test: add a device with "exp_alias" tag in fstab
Change-Id: If55730cedd21ec5a40137d2eb723a97df498c9cb
Signed-off-by: Daeho Jeong <daehojeong@google.com>