jffs2: check the validity of dstlen in jffs2_zlib_compress()

KASAN reports a BUG when download file in jffs2 filesystem.It is
because when dstlen == 1, cpage_out will write array out of bounds.
Actually, data will not be compressed in jffs2_zlib_compress() if
data's length less than 4.

[  393.799778] BUG: KASAN: slab-out-of-bounds in jffs2_rtime_compress+0x214/0x2f0 at addr ffff800062e3b281
[  393.809166] Write of size 1 by task tftp/2918
[  393.813526] CPU: 3 PID: 2918 Comm: tftp Tainted: G    B           4.9.115-rt93-EMBSYS-CGEL-6.1.R6-dirty #1
[  393.823173] Hardware name: LS1043A RDB Board (DT)
[  393.827870] Call trace:
[  393.830322] [<ffff20000808c700>] dump_backtrace+0x0/0x2f0
[  393.835721] [<ffff20000808ca04>] show_stack+0x14/0x20
[  393.840774] [<ffff2000086ef700>] dump_stack+0x90/0xb0
[  393.845829] [<ffff20000827b19c>] kasan_object_err+0x24/0x80
[  393.851402] [<ffff20000827b404>] kasan_report_error+0x1b4/0x4d8
[  393.857323] [<ffff20000827bae8>] kasan_report+0x38/0x40
[  393.862548] [<ffff2...