fs: fat: search file should not allocate cluster

Searching for a file is not a write operation. So it should not lead to the
allocation of a new cluster to the directory.

If we reuse deleted entries, we might not even use the new cluster and due
to not flushing it the directory could be corrupted.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>