directory extents
The directory extent list does not have to be a continuous list of data
blocks. When GRUB tries to read a non-existant member of the list,
grub_xfs_read_file() will return a block of zero'ed memory. Checking for
a zero'ed magic number is sufficient to skip this non-existant data block.
(cherry picked from commit 242fa05f2412202b241bbea01dc2bd69214b2e33)
