f2fs: reserve bits for fs-verity

Reserve an F2FS feature flag and inode flag for fs-verity.  This is an
in-development feature that is planned be discussed at LSF/MM 2018 [1].
It will provide file-based integrity and authenticity for read-only
files.  Most code will be in a filesystem-independent module, with
smaller changes needed to individual filesystems that opt-in to
supporting the feature.  An early prototype supporting F2FS is available
[2].  Reserving the F2FS on-disk bits for fs-verity will prevent users
of the prototype from conflicting with other new F2FS features.

Note that we're reserving the inode flag in f2fs_inode.i_advise, which
isn't really appropriate since it's not a hint or advice.  But
->i_advise is already being used to hold the 'encrypt' flag; and F2FS's
->i_flags uses the generic FS_* values, so it seems ->i_flags can't be
used for an F2FS-specific flag without additional work to remove the
assumption that ->i_flags uses the generic flags namespace.

[1] https://marc.info/?l=linux-fsdevel&m=151690752225644
[2] https://git.kernel.org/pub/scm/linux/kernel/git/mhalcrow/linux.git/log/?h=fs-verity-dev



Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>