EXTLINUX: Initial XFS filesystem support

EXTLINUX: Initial XFS filesystem support

XFS filesystems are divided into a number of equally sized chunks called
Allocation Groups (AGs). Each AG can almost be thought of as an
individual filesystem that maintains its own's space usage.

Each AG has the following structure:
     - A super block describing the overall filesystem info
     - Free space management
     - Inode allocation and tracking

So, due to historical reasons (SGI IRIX's design of disk layouts), the
first sector in the primary AG on XFS filesystems contains the
superblock, which is a problem with bootloaders that rely on BIOSes (that
load VBRs which are located in the first sector of the partition).

This patch adds support to the EXTLINUX installer to be able to install
Syslinux in an XFS partition, but with a limitation (temporarily) of
relying on the filesystem block size being 4+ KiB, otherwise there won't
be enough space to store the Syslinux boot sector in the blank area around
the 2048 to 4096 bytes of the primary AG/superblock.

Signed-off-by: Paulo Alcantara <pcacjr@zytor.com>