locks: close potential race in lease_get_mtime

lease_get_mtime is called without the i_lock held, so there's no
guarantee about the stability of the list. Between the time when we
assign "flock" and then dereference it to check whether it's a lease
and for write, the lease could be freed.

Ensure that that doesn't occur by taking the i_lock before trying
to check the lease.

Cc: J. Bruce Fields <[email protected]>
Signed-off-by: Jeff Layton <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>

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