The commit 315db9a05b7a ("cifs: fix leak in cifs_smb3_do_mount() ctx")
revealed an existing bug when mounting shares that contain a prefix
path or DFS links.

cifs_setup_volume_info() requires the @devname to contain the full
path (UNC + prefix) to update the fs context with the new UNC and
prepath values, however we were passing only the UNC
path (old_ctx->UNC) in @device thus discarding any prefix paths.

Instead of concatenating both old_ctx->{UNC,prepath} and pass it in
@devname, just keep the dup'ed values of UNC and prepath in
cifs_sb->ctx after calling smb3_fs_context_dup(), and fix
smb3_parse_devname() to correctly parse and not leak the new UNC and
prefix paths.

Cc: <redacted> # v5.11+
Fixes: 315db9a05b7a ("cifs: fix leak in cifs_smb3_do_mount() ctx")
Signed-off-by: Paulo Alcantara (SUSE) <redacted>
---
 fs/cifs/cifsfs.c     |  8 +-------
 fs/cifs/connect.c    | 24 ++++++++++++++++++------
 fs/cifs/fs_context.c |  4 ++++
 3 files changed, 23 insertions(+), 13 deletions(-)

diff --git a/fs/cifs/cifsfs.c b/fs/cifs/cifsfs.c
index 8a6894577697..d7ea9c5fe0f8 100644
--- a/fs/cifs/cifsfs.c
+++ b/fs/cifs/cifsfs.c

@@ -863,13 +863,7 @@ cifs_smb3_do_mount(struct file_system_type *fs_type,
 		goto out;
 	}
 
-	/* cifs_setup_volume_info->smb3_parse_devname() redups UNC & prepath */
-	kfree(cifs_sb->ctx->UNC);
-	cifs_sb->ctx->UNC = NULL;
-	kfree(cifs_sb->ctx->prepath);
-	cifs_sb->ctx->prepath = NULL;
-
-	rc = cifs_setup_volume_info(cifs_sb->ctx, NULL, old_ctx->UNC);
+	rc = cifs_setup_volume_info(cifs_sb->ctx, NULL, NULL);