ANDROID: fs: Restore vfs_path_lookup() export

Partial revert of 197df04c ("rename user_path_umountat() to
user_path_mountpoint_at()"), to restore access to vfs_path_lookup()
without including fs/internal.h, as needed by sdcardfs.

Bug: 120446149
Change-Id: I757f2df9f4dcc66f633939e7833e6fa2ac0ff4f8
Signed-off-by: Alistair Strachan <astrachan@google.com>