ext4: make some functions static in extents.c

ext4: make some functions static in extents.c

Make the following functions static since they're only used in
extents.c:

	__ext4_ext_dirty()
	ext4_can_extents_be_merged()
	ext4_collapse_range()
	ext4_insert_range()

Also remove the prototype for ext4_ext_writepage_trans_blocks(), as this
function is not defined anywhere.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20191231180444.46586-5-ebiggers@kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
Reviewed-by: Jan Kara <jack@suse.cz>