lockless get_write_access/deny_write_access

new helpers: atomic_inc_unless_negative()/atomic_dec_unless_positive()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --cc fs/namei.c
Simple merge

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

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