libfdisk: don't use blkdev ioctls for regular files

libfdisk: don't use blkdev ioctls for regular files

$ fdisk <diskimage>

open("sdc.img", O_RDONLY|O_CLOEXEC)     = 3
ioctl(3, BLKSSZGET, 0x7ffcf51357c4)     = -1 ENOTTY (Inappropriate ioctl for device)
ioctl(3, BLKGETSIZE64, 0x7ffcf51357b0)  = -1 ENOTTY (Inappropriate ioctl for device)
ioctl(3, BLKGETSIZE, 0x7ffcf51356c8)    = -1 ENOTTY (Inappropriate ioctl for device)
ioctl(3, FDGETPRM, 0x7ffcf51356d0)      = -1 ENOTTY (Inappropriate ioctl for device)
ioctl(3, HDIO_GETGEO, 0x7ffcf5135790)   = -1 ENOTTY (Inappropriate ioctl for device)

Signed-off-by: Karel Zak <kzak@redhat.com>