[PATCH v4 4/5] pstore: add pmsg

A secured user-space accessible pstore object. Writes
to /dev/pmsg0 are appended to the buffer, on reboot
the persistent contents are available in
/sys/fs/pstore/pmsg-ramoops-[ID].

One possible use is syslogd, or other daemon, can
write messages, then on reboot provides a means to
triage user-space activities leading up to a panic
as a companion to the pstore dmesg or console logs.

Signed-off-by: Mark Salyzyn <salyzyn@android.com>

v2: switch from snprintf to scnprintf
v3: split out prz_ok checking into PATCH 3/5
    replace pmsg_lseek with noop_llseek
    use pr_fmt() macro
    make write atomic and use a vmalloc'd bounce buffer
v4: use mutex_lock instead of spin_lock.

Change-Id: I82a2a9a989d7583c5fcb65ff520027dc3a034a4c