fs/9p: remove walk and inode allocation from symlink

Symlink had a bunch of extra operations which essentially
end up discarded.  It was walking the fid to the new file and
creating an inode for it, but those semantics are part of
tsymlink.  This did prepopulate the cache, but that also seems
potentially unnecessary and frought with peril.

Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>