nfsd: don't break lease on CLAIM_DELEGATE_CUR

CLAIM_DELEGATE_CUR is used in response to a broken lease; allowing it
to break the lease and return EAGAIN leaves the client unable to make
progress in returning the delegation

nfs4_get_vfs_file() now takes struct nfsd4_open for access to the
claim type, and calls nfsd_open() with NFSD_MAY_NOT_BREAK_LEASE when
claim type is CLAIM_DELEGATE_CUR

Signed-off-by: Casey Bodley <cbodley@citi.umich.edu>
Cc: stable@kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>