rename setlease to generic_setlease

Make it a little more clear that this is the default implementation for
the setleast operation.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Cc: Steven Whitehouse <swhiteho@redhat.com>
Acked-by: "J. Bruce Fields" <bfields@fieldses.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>