ext4: don't build the fsverity work handler for !CONFIG_FS_VERITY

ext4: don't build the fsverity work handler for !CONFIG_FS_VERITY

Use IS_ENABLED to disable this code, leading to a slight size reduction:

   text	   data	    bss	    dec	    hex	filename
   4121	    376	     16	   4513	   11a1	fs/ext4/readpage.o.old
   4030	    328	     16	   4374	   1116	fs/ext4/readpage.o

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Link: https://lore.kernel.org/r/20260128152630.627409-4-hch@lst.de
Signed-off-by: Eric Biggers <ebiggers@kernel.org>