NFSD: Fix a memory leak in nfsd4_create_session

If failed after calling alloc_session but before init_session, nfsd will call __free_session to
free se_slots in session. But, session->se_fchannel.maxreqs is not initialized (value is zero).
So that, the memory malloced for slots will be lost in free_session_slots for maxreqs is zero.

This path sets the information for channel in alloc_session after mallocing slots succeed,
instead in init_session.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>