freevxfs: Add missing lock_kernel() to vxfs_readdir

Commit 7b2fd697427e73c81d5fa659efd91bd07d303b0e in the historical GIT tree
stopped calling the readdir member of a file_operations struct with the big
kernel lock held, and fixed up all the readdir functions to do their own
locking.  However, that change added calls to unlock_kernel() in
vxfs_readdir, but no call to lock_kernel().  Fix this by adding a call to
lock_kernel().

Signed-off-by: Josh Triplett <josh@freedesktop.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>