mm: rcu-protected get_mm_exe_file()

mm: rcu-protected get_mm_exe_file()

This patch removes mm->mmap_sem from mm->exe_file read side.
Also it kills dup_mm_exe_file() and moves exe_file duplication into
dup_mmap() where both mmap_sems are locked.

[[email protected]: fix comment typo]
Signed-off-by: Konstantin Khlebnikov <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>