[PATCH v2 wireless] wifi: rtw88: debugfs: don't vzalloc(0) when rsvd_page is read unconfigured

Reading the rsvd_page debugfs entry before writing a page offset and
count to it calls vzalloc() with a size of zero, because
rsvd_page.page_num defaults to 0.  vmalloc warns about this:

  WARNING: mm/vmalloc.c:4019 at __vmalloc_node_range_noprof+0x9ac/0xa30
  ...
  Call Trace:
   __vmalloc_node_noprof+0x4c/0x70
   rtw_debugfs_get_rsvd_page+0x4c/0x110 [rtw88_core]
   seq_read_iter+0x132/0x4b0
   seq_read+0x12e/0x1c0
   full_proxy_read+0x6f/0xc0
   vfs_read+0xdd/0x490

rtw_fw_dump_fifo() would reject the zero size anyway, but only after
the allocation.  Check for a zero buffer size first and print a short
usage hint instead of allocating.

Fixes: e3037485c68e ("rtw88: new Realtek 802.11ac driver")
Signed-off-by: Abdurrahman Karadag <redacted>
---
v2: no code change. Add the wireless target tree to the subject and Cc
    the maintainers/blamed authors that v1 missed.

 drivers/net/wireless/realtek/rtw88/debug.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/net/wireless/realtek/rtw88/debug.c b/drivers/net/wireless/realtek/rtw88/debug.c
index b67d69b..84e45af 100644
--- a/drivers/net/wireless/realtek/rtw88/debug.c
+++ b/drivers/net/wireless/realtek/rtw88/debug.c

@@ -315,6 +315,11 @@ static int rtw_debugfs_get_rsvd_page(struct seq_file *m, void *v)
 	int i;
 	int ret;
 
+	if (!buf_size) {
+		seq_puts(m, "usage: echo <page_offset> <page_num> > rsvd_page\n");
+		return 0;
+	}
+
 	buf = vzalloc(buf_size);
 	if (!buf)
 		return -ENOMEM;

-- 
2.55.0