coredump: plug a memory leak situation on dump_seek()

After having started writing the coredump, if filesystem reports an error
anytime while writing part of the core file, we would leak a memory page
when bailing out.

Signed-off-by: André Goddard Rosa <andre.goddard@gmail.com>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Roland McGrath <roland@redhat.com>
Cc: WANG Cong <amwang@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>