libext2fs: have UNIX IO manager use pread64/pwrite64

Commit baa3544609da3c ("libext2fs: have UNIX IO manager use
pread/pwrite) causes a breakage on 32-bit systems where off_t is
32-bits for file systems larger than 4GB.  Fix this by using
pread64/pwrite64 if possible, and if pread64/pwrite64 is not present,
using pread/pwrite only if the size of off_t is at least as big as
ext2_loff_t.

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