fs/buffer.c: make __getblk_slow() static

__getblk_slow() was exported to modules in commit 3b5e6454
("fs/buffer.c: support buffer cache allocations with gfp modifiers").
This seems to have been a mistake, as no users were introduced nor was
the function declared in a header.  Change it back to 'static'.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>