ext4: extent macros cleanup

Use the EXT_LAST_INDEX macro; that's what it's there for.

Clean up ext4_ext_ext_grow_indepth() so the correct EXT_FIRST_INDEX or
EXT_FIRST_MACRO is used as necessary.  The two macros are equivalent, so
the C will collapse the if statement out, but it makes the code much
more readable.

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Acked-by: Alex Tomas <alex@clusterfs.com>
Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Singed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>