hpfs_unlink() calls permission() prior to truncating the file.  HPFS
doesn't define a .permission method, so replace with explicit call to
generic_permission().

This is equivalent, except that devcgroup_inode_permission() and
security_inode_permission() are not called.

The truncation is just an implementation detail of the unlink, so
these security checks are unnecessary.

I suspect that even calling generic_permission() is unnecessary, since
we shouldn't mind if the file isn't writable.  But I leave that to the
maintainer to decide.
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
CC: Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz>