mm: gup: fix potential pgmap refcnt leak in __gup_device_huge()

mm: gup: fix potential pgmap refcnt leak in __gup_device_huge()

When failed to try_grab_page, put_dev_pagemap() is missed.  So pgmap
refcnt will leak in this case.  Also we remove the check for pgmap against
NULL as it's also checked inside the put_dev_pagemap().

[[email protected]: simplify, cleanup]
[[email protected]: fix return value]

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Miaohe Lin <[email protected]>
Fixes: 3faa52c03f44 ("mm/gup: track FOLL_PIN pages")
Reviewed-by: John Hubbard <[email protected]>
Reviewed-by: Claudio Imbrenda <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>