jbd2: simplify code flow in do_get_write_access()

needs_copy is set only in one place in do_get_write_access(), just move
the frozen buffer copying into that place and factor it out to a
separate function to make do_get_write_access() slightly more readable.

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