drivers: shs: Protect all file system operations using shs ep lock

This add synchronization between the file system operations and the
updates which occur within the shs workqueue. Fixes the following
when there are two instances of userspace handlers running and are
killed together-

<6> Unable to handle kernel paging request at virtual address ffffffbfadadadb4
<2> pc : __free_pages+0x24/0xc0
<2> lr : free_pages+0x38/0x48
<2> Call trace:
<2>  __free_pages+0x24/0xc0
<2>  free_pages+0x38/0x48
<2>  rmnet_shs_release_caps+0x9c/0xb0 [rmnet_shs]
<2>  close_pdeo+0x94/0x120
<2>  proc_reg_release+0x64/0x88
<2>  __fput+0xdc/0x1d8
<2>  ____fput+0x1c/0x28
<2>  task_work_run+0x48/0xd0
<2>  do_notify_resume+0x950/0x1160
<2>  work_pending+0x8/0x14

CRs-fixed: 2576578
Change-Id: I67d1fc4d1f3c93d4497e988c2118c410091f0dd2
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>