nfsd: Fix inconsistent assignment

Dereferenced pointer "dentry" without checking and assigned to inode
in the declaration.

(We could just delete the NULL checks that follow instead, as we never
get to the encode function in this particular case.  But it takes a
little detective work to verify that fact, so it's probably safer to
leave the checks in place.)

Cc: Steve French <smfltc@us.ibm.com>
Signed-off-by: Prasad V Potluri <pvp@us.ibm.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>