Fix "swapon: swapfile has holes" error in Linux kernel 5.7

[mnghsn]

When creating a swap file, dynamic space allocation such as using fallocate is not supported now.

swapon(8) says:

The swap file implementation in the kernel expects to be able to write to the file directly, without the assistance of the filesystem. This is a problem on files with holes or on copy-on-write files on filesystems like Btrfs.

Commands like cp(1) or truncate(1) create files with holes. These files will be rejected by swapon.

Preallocated files created by fallocate(1) may be interpreted as files with holes too depending of the filesystem. Preallocated swap files are supported on XFS since Linux 4.18.

The most portable solution to create a swap file is to use dd(1) and /dev/zero.

This PR changes the way to create swap file from fallocate to dd.

References:

• https://jlk.fjfi.cvut.cz/arch/manpages/man/swapon.8#Files_with_holes
• https://wiki.archlinux.org/index.php/Swap#Manually
• https://bugs.archlinux.org/task/66921