vfs: export check_sticky()

It's already duplicated in btrfs and about to be used in overlayfs too.

Move the sticky bit check to an inline helper and call the out-of-line
helper only in the unlikly case of the sticky bit being set.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

diff --cc fs/btrfs/ioctl.c
Simple merge

diff --cc fs/namei.c
Simple merge

diff --cc include/linux/fs.h
Simple merge