fs: track fl_owner for leases

Just like for other lock types we should allow different owners to have
a read lease on a file.  Currently this can't happen, but with the addition
of pNFS layout leases we'll need this feature.

Signed-off-by: Christoph Hellwig <hch@lst.de>

diff --cc fs/locks.c
Simple merge

diff --cc fs/nfsd/nfs4state.c
Simple merge