ANDROID: Fix incorrect namespacing for ANDROID_GKI_VFS_EXPORT_ONLY

When the ANDROID_GKI_VFS_EXPORT_ONLY was added, the namespace was
inconsistently named due to missing definition in fs/Makefile.
Consequently, two different errors were emitted by kernelci
showing that the namespace name is not consistent:

    "ERROR: modpost: module binfmt_misc uses symbol override_creds
    from namespace VFS_internal_I_am_really_a_filesystem_and_am_NOT_a_driver,
    but does not import it"

and

    "ERROR: modpost: module configfs uses symbol d_drop from namespace
    ANDROID_GKI_VFS_EXPORT_ONLY, but does not import it."

Fix via a partial cherry-pick of commit a38b207d4f4e ("ANDROID: GKI: set
vfs-only exports into their own namespace") from android15-6.6

Fixes: e0f526b01b ("ANDROID: GKI: add more vfs-only exports to their own namespace")
Signed-off-by: Todd Kjos <tkjos@google.com>
Change-Id: I93ecd265d2664eee048d4314f4f3e8628695dce6

fs/Makefile

@@ -6,6 +6,7 @@
 # Rewritten to use lists instead of if-statements.
 # 
 
+subdir-ccflags-y += -DANDROID_GKI_VFS_EXPORT_ONLY=VFS_internal_I_am_really_a_filesystem_and_am_NOT_a_driver
 
 obj-y :=	open.o read_write.o file_table.o super.o \
 		char_dev.o stat.o exec.o pipe.o namei.o fcntl.o \