libext2fs: fix write size in ext2fs_mmp_write

libext2fs: fix write size in ext2fs_mmp_write

Without this change, we will write data past the end of the
mmp buf.  Valgrind catches this:

==6373== Syscall param write(buf) points to unaddressable byte(s)
==6373==    at 0x362260E470: __write_nocancel (in /lib64/libpthread-2.12.2.so)
==6373==    by 0x41CF83: raw_write_blk (unix_io.c:255)
==6373==    by 0x41D2BC: unix_write_blk64 (unix_io.c:757)
==6373==    by 0x41A05D: ext2fs_mmp_write (mmp.c:130)
==6373==    by 0x40B0C9: do_set_mmp_value (set_fields.c:806)
==6373==    by 0x421B61: really_execute_command (execute_cmd.c:108)
==6373==    by 0x421C54: ss_execute_line (execute_cmd.c:234)
==6373==    by 0x403743: main (debugfs.c:2339)
==6373==  Address 0x63f000 is not stack'd, malloc'd or (recently) free'd

and in my testing it led to silent failures while writing the mmp
block in debugfs:

write(3, "xV4\22PMM\342\325V\274N\0\0\0\0host.name."..., 4096) = -1 EFAULT (Bad address)

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>