scsi: st: Convert convert get_user_pages() --> pin_user_pages()

scsi: st: Convert convert get_user_pages() --> pin_user_pages()

This code was using get_user_pages*(), in a "Case 1" scenario (Direct IO),
using the categorization from [1]. That means that it's time to convert the
get_user_pages*() + put_page() calls to pin_user_pages*() +
unpin_user_pages() calls.

There is some helpful background in [2]: basically, this is a small part of
fixing a long-standing disconnect between pinning pages, and file systems'
use of those pages.

Note that this effectively changes the code's behavior as well: it now
ultimately calls set_page_dirty_lock(), instead of SetPageDirty().This is
probably more accurate.

As Christoph Hellwig put it, "set_page_dirty() is only safe if we are
dealing with a file backed page where we have reference on the inode it
hangs off." [3]

Also, this deletes one of the two FIXME comments (about refcounting),
because there is nothing wrong with the refcounting at this point.

[1] Documentation/core-api/pin_user_pages.rst

[2] "Explicit pinning of user-space pages":
    https://lwn.net/Articles/807108/

[3] https://lore.kernel.org/r/20190723153640.GB720@lst.de

Link: https://lore.kernel.org/r/20200526182709.99599-1-jhubbard@nvidia.com
Cc: "Kai Mäkisara (Kolumbus)" <kai.makisara@kolumbus.fi>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: James E.J. Bottomley <jejb@linux.ibm.com>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Cc: linux-scsi@vger.kernel.org
Acked-by: Kai Mäkisara <kai.makisara@kolumbus.fi>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>