e2fsck: fix corruption of Hurd filesystems

e2fsck: fix corruption of Hurd filesystems

Previously, e2fsck accessed the field osd2.linux2.l_i_file_acl_high
field without checking that the filesystem is indeed created for
Linux.  This lead to e2fsck constantly complaining about certain
nodes:

i_file_acl_hi for inode XXX (/dev/console) is 32, should be zero.

By "correcting" this problem, e2fsck would clobber the field
osd2.hurd2.h_i_mode_high.

Properly guard access to the OS dependent fields.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>