staging: lustre: remove kmalloc from fld_proc_hash_seq_write

This patch simplifies the fld_proc_hash_seq_write function
by removing the dynamic memory allocation.
The longest fh_name used so far in lustre is 4 characters.
We use a 8 bytes variable to be on the safe side.

Signed-off-by: Tristan Lelong <tristan@lelong.xyz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>