smack: Initialize the in-memory inode in smack_inode_init_security()

Currently, Smack initializes in-memory new inodes in three steps. It first
sets the xattrs in smack_inode_init_security(), fetches them in
smack_d_instantiate() and finally, in the same function, sets the in-memory
inodes depending on xattr values, unless they are in specially-handled
filesystems.

Other than being inefficient, this also prevents filesystems not supporting
xattrs from working properly since, without xattrs, there is no way to pass
the label determined in smack_inode_init_security() to
smack_d_instantiate().

Since the LSM infrastructure allows setting and getting the security field
without xattrs through the inode_setsecurity and inode_getsecurity hooks,
make the inode creation work too, by initializing the in-memory inode
earlier in smack_inode_init_security().

Also mark the inode as instantiated, to prevent smack_d_instantiate() from
overwriting the security field. As mentioned above, this potentially has
impact for inodes in specially-handled filesystems in
smack_d_instantiate(), if they are not handled in the same way in
smack_inode_init_security().

Filesystems other than tmpfs don't call security_inode_init_security(), so
they would be always initialized in smack_d_instantiate(), as before. For
tmpfs, the current behavior is to assign to inodes the label '*', but
actually that label is overwritten with the one fetched from the SMACK64
xattr, set in smack_inode_init_security() (default: '_').

Initializing the in-memory inode is straightforward: if not transmuting,
nothing more needs to be done; if transmuting, overwrite the current inode
label with the one from the parent directory, and set SMK_INODE_TRANSMUTE.
Finally, set SMK_INODE_INSTANT for all cases, to mark the inode as
instantiated.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>