staging: ashmem: Avoid deadlock between read and mmap calls

Avoid holding ashmem_mutex across code that can page fault.  Page faults
grab the mmap_sem for the process, which are also held by mmap calls
prior to calling ashmem_mmap, which locks ashmem_mutex.  The reversed
order of locking between the two can deadlock.

The calls that can page fault are read() and the ASHMEM_SET_NAME and
ASHMEM_GET_NAME ioctls.  Move the code that accesses userspace pages
outside the ashmem_mutex.

Cc: Colin Cross <[email protected]>
Cc: Android Kernel Team <[email protected]>
Signed-off-by: Todd Poynor <[email protected]>
[jstultz: minor commit message tweaks]
Signed-off-by: John Stultz <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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