leases: move lock allocation earlier in generic_setlease()

In generic_setlease(), the struct file_lock is allocated after tests for the
presence of conflicting readers/writers is done, despite the fact that the
allocation might block; this patch moves the allocation earlier.  A subsequent
set of patches will rely on this behavior to properly serialize between a
modified __break_lease() and generic_setlease().

Signed-off-by: David M. Richter <richterd@citi.umich.edu>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>