Increase minimum XFS size to 300 MiB (#217)

As documented in the previous commit xfsprogs >= 5.19.0 refuses to
create an XFS file system smaller than 300 MiB.

    $ truncate -s $((300*1024*1024-1)) test.img
    $ ls -l test.img
    -rw-r--r--    1 auser    auser   314572799 Dec 21 11:01 test.img
    $ mkfs.xfs -V
    mkfs.xfs version 6.0.0
    $ mkfs.xfs test.img
    Filesystem must be larger than 300MB.
    ...
    $ echo $?
    1

Successfully create an XFS file system at minimum size of 300 MiB.

    $ truncate -s $((300*1024*1024)) test.img
    $ ls -l test.img
    -rw-r--r--    1 auser    auser   314572800 Dec 21 11:05 test.img
    $ mkfs.xfs test.img
    ...
    $ echo $?
    0
    $ blkid test.img
    test.img: UUID="..." BLOCK_SIZE="512" TYPE="xfs"

Increase the GParted minimum XFS size to 300 MiB.  For simplicity and
because the XFS developers said of smaller XFS file systems [1]:
    "are known to have performance and redundancy problems that are not
    present on the volume sizes that XFS is best at handling"
regardless of the version of mkfs.xfs used to create that XFS then apply
to all versions of xfsprogs.

[1] https://git.kernel.org/pub/scm/fs/xfs/xfsprogs-dev.git/commit/?id=6e0ed3d19c54603f0f7d628ea04b550151d8a262
    mkfs: stop allowing tiny filesystems

Closes #217 - GitLab CI test job failing with new mkfs.xfs error
              "Filesystem must be larger than 300MB."

src/xfs.cc

@@ -92,8 +92,8 @@ FS xfs::get_filesystem_support()
 		fs .online_grow = fs .grow ;
 #endif
 
-	// Official minsize = 16MB, but the smallest xfs_repair can handle is 32MB.
+	// From xfsprogs 5.19.0 the smallest creatable file system is 300 MiB.
-	fs_limits.min_size = 32 * MEBIBYTE;
+	fs_limits.min_size = 300 * MEBIBYTE;
 
 	return fs ;
 }