vfs: use kmalloc() to allocate fdmem if possible

vfs: use kmalloc() to allocate fdmem if possible

Use kmalloc() to allocate fdmem if possible.

vmalloc() is used as a fallback solution for fdmem allocation.  A new
helper function __free_fdtable() is introduced to reduce the lines of
code.

A potential bug, vfree() a memory allocated by kmalloc(), is fixed.

[[email protected]: use __GFP_NOWARN, uninline alloc_fdmem() and free_fdmem()]
Signed-off-by: Changli Gao <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Jiri Slaby <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Avi Kivity <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>