losetup: don't require 512-byte aligned offsets

losetup: don't require 512-byte aligned offsets

Kernel aligns the device size, but the offset where the device starts
is not required to be aligned.

 # losetup --offset 32 -f file.img

is just fine, the final size of the look device will be (in sectors)

  (backing_file_size - offset) >> 9

so we have to do the same in userspace when we check for successful
set capacity ioctl.

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