knfsd: query filesystem for NFSv4 getattr of FATTR4_MAXNAME

mainline: a16e92ed

Without this we always return 2^32-1 as the the maximum namelength.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Andreas Gruenbacher <agruen@suse.de>
CC: Oliver Pinter <oliver.pntr@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>