proc: pagemap: Hold mmap_sem during page walk

proc: pagemap: Hold mmap_sem during page walk

If the mmap_sem is not held while we walk_page_range(), then
it is possible for find_vma() to race with a remove_vma_list()
caused by do_munmap() (or others).

Unable to handle kernel paging request at virtual address 6b6b6b5b
Internal error: Oops: 5 [#1] PREEMPT
CPU: 0    Not tainted  (2.6.32.9-27154-ge3e6e27 #1)
PC is at find_vma+0x40/0x7c
LR is at walk_page_range+0x70/0x230
pc : [<c00aa3ac>]    lr : [<c00b298c>]    psr: 20000013
sp : c6aa9eb8  ip : 6b6b6b53  fp : c6a58f60
r10: c7e1d1b8  r9 : 0001bca0  r8 : 47000000
r7 : c6aa9f80  r6 : c6aa8000  r5 : 46fbd000  r4 : 6b6b6b6b
r3 : c7ca4820  r2 : 6b6b6b6b  r1 : 46fbd000  r0 : c70e3e40
Flags: nzCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment user
Control: 10c5787d  Table: 26574019  DAC: 00000015

[<c00aa3ac>] (find_vma+0x40/0x7c) from [<c00b298c>] (walk_page_range+0x70/0x230)
[<c00b298c>] (walk_page_range+0x70/0x230) from [<c00f5d3c>] (pagemap_read+0x1a4/0x278)
[<c00f5d3c>] (pagemap_read+0x1a4/0x278) from [<c00bac40>] (vfs_read+0xa8/0x150)
[<c00bac40>] (vfs_read+0xa8/0x150) from [<c00bad94>] (sys_read+0x3c/0x68)
[<c00bad94>] (sys_read+0x3c/0x68) from [<c0026f00>] (ret_fast_syscall+0x0/0x2c)
Code: 98bd8010 e5932004 e3a00000 ea000008 (e5124010)

Signed-off-by: San Mehat <san@google.com>
CC: Brian Swetland <swetland@google.com>
CC: Matt Mackall <mpm@selenic.com>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: Linus Torvalds <torvalds@linux-foundation.org>