9p: fix access mode flags being ORed instead of replaced

In the Linux kernel, the following vulnerability has been resolved:

9p: fix access mode flags being ORed instead of replaced

Since commit 1f3e4142c0eb ("9p: convert to the new mount API"),

v9fs_apply_options() applies parsed mount flags with |= onto flags

already set by v9fs_session_init(). For 9P2000.L, session_init sets

V9FS_ACCESS_CLIENT as the default, so when the user mounts with

"access=user", both bits end up set. Access mode checks compare

against exact values, so having both bits set matches neither mode.

This causes v9fs_fid_lookup() to fall through to the default switch

case, using INVALID_UID (nobody/65534) instead of current_fsuid()

for all fid lookups. Root is then unable to chown or perform other

privileged operations.

Fix by clearing the access mask before applying the user's choice.