hw/9pfs: fix data race in v9fs_mark_fids_unreclaim()

A data race between v9fs_mark_fids_unreclaim() and v9fs_path_copy()
causes an inconsistent read of fidp->path. In v9fs_path_copy(), the
path size is set before the data pointer is allocated, creating a
window where size is non-zero but data is NULL.

v9fs_co_open2() holds a write lock during path modifications,
but v9fs_mark_fids_unreclaim() was not acquiring a read
lock, allowing it to race.

Fix by holding the path read lock during FID table iteration.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3300
Signed-off-by: Richie Buturla <richie@linux.ibm.com>
Link: https://lore.kernel.org/qemu-devel/20260211154450.254338-1-richie@linux.ibm.com/
Fixes: 7a46274529 ("hw/9pfs: Add file descriptor reclaim support")
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>

hw/9pfs/9p.c

@@ -560,6 +560,7 @@ static int coroutine_fn v9fs_mark_fids_unreclaim(V9fsPDU *pdu, V9fsPath *path)
             sizeof(V9fsFidState *), 1);
     gint i;
 
+    v9fs_path_read_lock(s);
     g_hash_table_iter_init(&iter, s->fids);
 
     /*
@@ -580,6 +581,7 @@ static int coroutine_fn v9fs_mark_fids_unreclaim(V9fsPDU *pdu, V9fsPath *path)
             g_array_append_val(to_reopen, fidp);
         }
     }
+    v9fs_path_unlock(s);
 
     for (i = 0; i < to_reopen->len; i++) {
         fidp = g_array_index(to_reopen, V9fsFidState*, i);