ext4: remove (almost) unused variables from ext4_expand_extra_isize_ea()

'start' variable is completely unused in ext4_expand_extra_isize_ea().
Variable 'first' is used only once in one place. So just remove them.
Variables 'entry' and 'last' are only really used later in the function
inside a loop. Move their declarations there.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>