new helper: file_inode(file)

[alex: trimmed down to modify f2fs and linux/fs.h]

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Alejandro Martinez Ruiz <alex@nowcomputing.com>
Fix compiling error

This commit fixes the following error:

kernel/sched/core.c:2356:15: error: redefinition of ‘avg_nr_running’
 unsigned long avg_nr_running(void)
               ^
kernel/sched/core.c:2334:15: note: previous definition of ‘avg_nr_running’ was here
 unsigned long avg_nr_running(void)