fs/proc/task_mmu: remove conversion of seq_file position to unsigned

fs/proc/task_mmu: remove conversion of seq_file position to unsigned

Back in 2.6 era, last_addr used to be stored in seq_file->version
variable, which was unsigned long. As a result, sentinels to represent
gate vma and end of all vmas used unsigned values. In more recent
kernels we don't used seq_file->version anymore and therefore conversion
from loff_t into unsigned type is not needed. Similarly, sentinel values
don't need to be unsigned. Remove type conversion for set_file position
and change sentinel values to signed.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>