sftp: fix post transfer copies failing with 0 size when using set_modtime=false
sftp: fix post transfer copies failing with 0 size when using set_modtime=false

Before this change we early exited the SetModTime call which means we
skipped reading the info about the file.

This change reads info about the file in the SetModTime call even if
we are skipping setting the modtime.

See: https://forum.rclone.org/t/sftp-and-set-modtime-false-error/16362

ncw/rclone,ncw/rclone,ncw/rclone,ncw/rclone