knfsd: query filesystem for NFSv4 getattr of FATTR4_MAXNAME

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

Thanks to Andreas Gruenbacher for bug report and testing.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Cc: Andreas Gruenbacher <agruen@suse.de>