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/[email protected]

Link: https://lore.kernel.org/r/[email protected]
Cc: "Kai Mäkisara (Kolumbus)" <[email protected]>
Cc: Bart Van Assche <[email protected]>
Cc: James E.J. Bottomley <[email protected]>
Cc: Martin K. Petersen <[email protected]>
Cc: [email protected]
Acked-by: Kai Mäkisara <[email protected]>
Signed-off-by: John Hubbard <[email protected]>
Signed-off-by: Martin K. Petersen <[email protected]>

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