swapfile: fix name leak in swapoff

swapfile: fix name leak in swapoff

There's a name leak introduced by commit 91a27b2a7567 ("vfs: define
struct filename and have getname() return it").  Add the missing
putname.

[[email protected]: cleanup]
Signed-off-by: Xiaotian Feng <[email protected]>
Reviewed-by: Jeff Layton <[email protected]>
Cc: Al Viro <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>